You might not immediately think of Disney+ as a go-to for documentaries, but there’s more than you think there are.
Whether you’re looking for a behind-the-scenes look at Obi-Wan Kenobi or Marvel’s big action scenes, or want to learn about where we’re at with climate change (spoiler: it’s not great), the streaming platform has many really strong documentaries on offer. Don’t forget there’s a whole National Geographic channel alongside the stash of Disney classics and Marvel menu, and there’s a starting lineup of ESPN’s 30 for 30 long-running documentary series — including a truly exceptional Bruce Lee film.
Here are the best documentaries on Disney+, both films and series, that will give you a closer look at the entertainment we inhale, the superheroes we fly with, and the planet we’re constantly begging world leaders to save.
Before we start, I’ve left out Taylor Swift’s Folklore: The Long Pond Studio Sessions and Billie Eilish’s Happier than Ever: A Love Letter to Los Angeles, as they’re more concert films, but they’re both worth watching as they’re both beautifully shot.
1. Be Water
Credit: Robert Clouse Concord /Warner Bros
How much do you actually know about Bruce Lee? Directed by Bao Ngyuen as part of ESPN’s long-running “30 for 30” documentary series, Be Water chronicles the tragically short but nonetheless influential life of the actor and martial arts expert. The story begins in 1971, when Lee had returned to his parents’ homeland of Hong Kong from America. There, he completed four films in two years, including the iconic Enter the Dragon. Tragically, Lee died just before it was released, at 32 years old.
Be Water explores Lee’s struggle to find community and industry support amid a time of blatantly racist representation of Asian Americans in Hollywood. Alongside the second-class treatment and pay that non-white actors received, most roles were played by white actors — take John Wayne as Genghis Khan or Mickey Rooney’s horrifying performance in Breakfast at Tiffany’s, for example. “When you saw Asian Americans on screen, they were usually crazed, maniacal, robotic stereotypes or serving white folks,” says cultural critic and writer Jeff Chang. The documentary speaks to how Lee was “rejected by Hollywood,” as his wife Linda Lee Cadwell says, “and so he had to show them that an Asian could be a strong leading man.” Lee refused to play roles that were demeaning to Chinese people or a “novelty.” So he started creating his own in Hong Kong.
The story is primarily told through old interviews with Lee and through those who knew him best — one of the most poignant features of the film is the access to Lee’s writings over the years, which are beautifully read by his daughter, Shannon. His poignant reflections on racism in America and Hollywood, as well as on humanity and expressing your true self, are strikingly relevant today.
How to watch: Be Water is now streaming on Disney+.
2. Free Solo
Credit: National Geographic / Jimmy Chin
A truly staggering feat in front of and behind the camera, Free Solo will deprive you of your fingernails (it’s that nervous-making). This Oscar-winning documentary is a stunning portrait of free soloist climber Alex Honnold, who is training and preparing to climb the 3,000-feet-high El Capitan in Yosemite National Park, “the most impressive wall on Earth,” with no rope. His painstakingly planned journey is boldly captured by documentary maker E. Chai Vasarhelyi and photographer/mountaineer Jimmy Chin, along with a team of filmmakers/professional climbers. “I’ve always been conflicted about shooting a film about free soling just because it’s so dangerous,” says Chin. “It’s hard to not imagine your friend, Alex, soloing something that’s extremely dangerous and you’re making a film about it, which might put undue pressure on him to do something, and him falling through the frame to his death.”
You’ll follow Honnold as he plans every move before heading up the wall of El Cap without that all-important rope. “There’s no margin for error,” big-wall free climbing legend Tommy Caldwell says in the doc of his friend’s task. “Imagine an Olympic gold medal level athletic achievement that if you don’t get that golden medal you’re gonna die. That’s pretty much what free soloing El Cap is like. You have to do it perfectly.”
How to watch: Free Solo is now streaming on Disney+.
3. Get Back
Credit: Linda McCartney / Apple Corps
Lord of the Rings director Peter Jackson was allowed access to 50 hours of “lost” footage from the 1970 documentary by Michael Lindsay-Hogg, which chronicled the making of The Beatles’ final album, Let It Be. Painstakingly trawling through it to build a new, vivid portrait of the legendary band in their last recording days together, Jackson crafted Get Back, a stunning three-part series for Disney+. As Mashable’s Adam Rosenberg writes in his review, “Jackson saw something in the vaulted studio footage that painted a different picture of the Let It Be sessions than anyone outside The Beatles organization had ever known. This wasn’t where The Beatles explicitly ended, Get Back contends. It’s four friends trying their damnedest to make their magical collaboration work one more time. There were aggravating factors during the recording that widened cracks that were already there, but the actual day-to-day work of building the next Beatles album was much more frequently a joyous and playful affair.” The moment in which you spy Linda McCartney and Yoko Ono having an inaudible chat in the background is worth it all.
if you’d like even more Beatles after Get Back, Paul McCartney sits down for a one-on-one interview with producer Rick Rubin in McCartney 3, 2, 1, also available on Disney+.
How to watch: Get Back is streaming on Disney+.
4. LA 92
Credit: Steve Grayson / WireImage
Directed by Daniel Lindsay and T. J. Martin, LA 92 examines in detail the 1992 Los Angeles riots, from the simmering lead-up through the riots themselves to the fallout. The documentary looks at police brutality and violent systemic racism leading to the events in April and May 1992. Rodney King, an unarmed Black man, was brutally beaten by four white Los Angeles police officers on March 3, 1991. The assault was filmed by a neighbour and brought to the attention of the country through the press. The officers involved — who had delivered 56 baton blows in the course of the beating — were acquitted. The documentary footage showing the officers confidently entering the court building before the ruling, combined with the reactions from the Black community when it’s delivered, is chilling, infuriating, and devastating. The lack of repercussions for those responsible for this violence against Black people sparked demonstrations, marches, and eventually riots, violence, and looting across the city of Los Angeles. LA 92 looks at these events from multiple perspectives, much of it from handheld footage across the city. It’s a hard documentary to watch, with significant moments of violence shown, but you should.
How to watch: LA 92 is now streaming on Disney+ (UK only).
5. Summer of Soul
Credit: Searchlight Pictures
An incredible directorial debut from Ahmir “Questlove” Thompson, Summer of Soul (…or, When the Revolution Could Not be Televised) centres around a truly pivotal music event in 1969 that took place 100 miles south of Woodstock: the Harlem Cultural Festival, a huge moment in Black history and culture. Stevie Wonder played. So did Nina Simone. Sly and the Family Stone, too, along with Gladys Knight and the Pips, B.B. King, The 5th Dimension, Mahalia Jackson, and many more. Footage from the six-week festival had never really been seen, until Questlove crafted this 2021 documentary. As Mashable’s Adam Rosenberg writes, “It’s a tragedy, and sadly a product of deep-seated racism in the U.S., that this beautiful expression of love and culture was completely hidden from public view for half a century. That sense of something lost is expressed implicitly again and again all throughout Summer of Soul.”
How to watch: Summer of Soul is now streaming on Disney+.
6. Obi-Wan Kenobi: A Jedi’s Return
Credit: Disney
If you want to see Hayden Christensen’s first day on set for Obi-Wan Kenobi, back in his Jedi robes as Anakin Skywalker after 17 years, you should watch the Disney+ documentary Obi-Wan Kenobi: A Jedi’s Return.
A behind-the-scenes look at the Star Wars series, the 60-minute film directed by David Gelb takes you through the making of Obi-Wan Kenobi with director Deborah Chow and stars Ewan McGregor, Hayden Christensen, Moses Ingram, Indira Varma, Kumail Nanjiani, Rupert Friend, Joel Edgerton, O’Shea Jackson Jr., Bonnie Piesse, and more.*
How to watch: Obi-Wan Kenobi: A Jedi’s Return is now streaming on Disney+.
7. Empire of Dreams: the Story of the Star Wars Trilogy
Credit: Sunset Boulevard / Corbis via Getty Images
A long(ish) time ago, in the galaxy you currently reside in, the Star Wars trilogy changed everything — and Empire of Dreams takes you behind the scenes of it all. This 2004 documentary obviously doesn’t cover the newer films, focusing instead on how the original films (Star Wars Episode IV: A New Hope, Episode V: The Empire Strikes Back, and Episode VI: Return of the Jedi) changed the way movies were made. Yes, it’s a little cheesy in its production (this is an early ‘00s doc), and it’s the official Lucasfilm story, so take it with a grain of salt and expect extreme reverence. But this means plenty of access to footage from the set (including the cast mucking about, and the incredibly tricky Dagobah swamp set), screen tests, early scripts, Ralph McQuarrie’s concept art, storyboards, and truly awesome videos of robotic droid testing, not to mention interviews with the big guns: George Lucas, Carrie Fisher, Harrison Ford, Mark Hamill, and more.
Did you know that Harrison Ford improvised the line “I know” in Empire? Did you know Darth Vader’s identity was kept a secret (with a false page included in the script) from everyone, including Mark Hamill, who was told just minutes before the scene — even James Earl Jones thought Vader was lying? Well, that’s what this documentary will tell you. Like Mashable’s Chris Taylor in his book (but not as in-depth), the documentary spends time exploring Star Wars as a cultural phenomenon, as well as the monumental wave of merchandise that came with the film’s popularity. Plus, you get to see the original trailer, featuring whatever footage the team had thrown together when they were still working on the movie.
How to watch: Empire of Dreams: The Story of the Star Wars Trilogy is streaming on Disney+.
8. The Rescue
Credit: National Geographic
In 2018, a rescue mission captured the attention of the world when 12 boys and their football coach were trapped deep inside a flooded cave in northern Thailand. Monsoon rains had cut off the group, who were stranded in the dark two kilometres into the cave — a labyrinth of tunnels completely filled with water. Against all odds, an incredibly high-risk, complicated rescue effort was formed, and The Rescue, from Free Solo directors E. Chai Vasarhelyi and Jimmy Chin, tracks it all.
“At its core, The Rescue is a story about a collection of people of all different nationalities, languages, and cultures working together to achieve a common goal,” said Vasarhelyi and Chin in a press statement. “In making the film, we were reminded of the beauty of humanity, especially after the last several years where the world has seemed more divided than ever before.” It’s a claustrophobic, stressful watch for the most part, and for everyone glued to the news when it was happening, it’ll bring back all those emotions of anticipation, stress, and finally, relief. The moment when British divers Richard Stanton and John Volanthen first find all 13 in the cave is astounding and deeply moving. But it’s just the beginning of a perilous mission.
How to watch: The Rescue is now streaming on Disney+.
9. Pride
Credit: FX
Covering six decades of resistance from the ’50s to the ’00s, the FX series Pride focuses on the personal stories of LGBTQ people in America, and the long (and continuing) fight for civil rights. Each episode has been made by different queer filmmakers and takes on a new decade, from the riots and revolutions of the ’60s to the underground ball scene of the ’80s, through to the “culture wars” of the ’90s and the new age of queer visibility in the 2000s. The six-part series uses actors to recreate some of the vibrant lives lived by queer people in these times, including Alia Shawkat, who plays Brooklyn-born employment lawyer and LGBTQ rights activist Madeleine Tress, reading out her memoirs. The episode on the ’70s made by Cheryl Dunye is a highlight, focusing on poet Audre Lorde and filmmaker Barbara Hammer. The series features events and people you might know, and others that have received less coverage — but each story is treated with respect, love, and admiration for those who have fought for equality, justice, and the right to live as their truest, most excellent selves.
How to watch: Pride is now streaming on Disney+.
10. Howard
Credit: Disney
If you grew up singing loudly in the car to “Part of Your World” from The Little Mermaid, “Be Our Guest” from Beauty and the Beast, or “Suddenly Seymour” from Little Shop of Horrors, you have lyricist Howard Ashman to thank. Ashman is the focus of Disney documentary Howard, which explores his early life, his career-defining work with Disney while battling AIDS, and his death at the age of 40. As Brooke Bajgrowicz writes for Mashable, “Whether you’re Disney-obsessed or not, you can get a lot out of Howard. If this film recognizes anything, it’s that story — even in the context of a documentary — is what will keep the audience engaged. And what better story is there to tell than one about a man who used songs to tell some of the greatest stories?”
There are a lot of documentaries on Disney+ about the makers of Disney films and the broader universe of the animation giant, naturally, so you could also check out Waking Sleeping Beauty, Frank and Ollie, The Imagineering Story, The Boys: The Sherman Brothers’ Story, Prop Culture, Behind the Attraction, The Pixar Story, and Inside Pixar, among others.
How to watch: Howard is now streaming on Disney+.
11. Assembled
Credit: Jasin Boland / Marvel Studios
Like the ol’ special features section of a DVD, Assembled gives you a look behind the scenes of the Marvel Cinematic Universe’s Phase Four releases. Sitting at five episodes so far, the series covers the making of Shang-Chi and the Legend of the Ten Rings, WandaVision, Loki, The Falcon and the Winter Soldier, What If…? , and Black Widow. You’ll be able to see how those ’60s Bewitched-style scenes in WandaVision were filmed with a live audience (and you’ll be able to see the sets and costumes in full colour), and watch Kathryn Hahn doing a full-on evil laugh take. You can watch Simu Liu in all those hectic action sequences in Shang-Chi. Loki-wise, you can see Tom Hiddleston’s early audition tapes for Thor and watch him making a speech for the crew in front of a giant tesseract cake.
If you want to watch a behind-the-scenes look at the earlier Marvel phases, check out the short documentary Assembling a Universe, which goes right back to the early Iron Man days.
How to watch: Assembled is now streaming on Disney+.
12. Behind the Mask
If you know nothing about Marvel Comics, want a quick overview in an hour, and don’t mind a bit of brand-led reverence, Behind the Mask is for you. Directed by Michael Jacobs, it’s probably more of a documentary for newcomers to the Marvel universe, but there’s enough compelling discussion, nostalgic interviews, and strong visuals to keep hardcore fans engaged — you’ll be revisiting a huge amount of Marvel comic frames, some of which are animated. If you don’t have the time or inclination to do what Douglas Wolk did and read every Marvel Comic ever (he’s in this film!), this might be just what you need to navigate the MCU.
Yes, it’s a documentary about Marvel created by Marvel and hosted on the streaming service that owns Marvel — so it’s more than a little reverent, but surprisingly self-aware when it comes to discussions around representation. But as well as having access to all of Marvel Comics’ major players, it’s still a solid crash course in the historical context amid which Marvel’s biggest characters were released to the world — the section on Black Panther is particularly strong.
If you want even more Marvel, check out Marvel 616, which looks at parts of the Marvel universe you might not know about, like the Japanese version of Spider-Man.
How to watch: Behind the Mask is now streaming on Disney+.
13. Jane
Credit: National Geographic Creative / Hugo Van Lawick
Made using over 100 hours of footage that had never been seen before, Jane is the quintessential portrait of Dr. Jane Goodall. Thought lost until its rediscovery in 2014, the footage was filmed by her ex-husband, the legendary wildlife photographer Hugo van Lawick in the ’60s, when he was sent on assignment to document Goodall’s work studying chimpanzees in Gombe, Tanzania. She was 26 at the time, recruited by Dr. Louis Leakey for the study in 1957, though she had no training or a scientific degree — yet her research was one of the first studies of its kind, to become the longest continuous study of any animal in their natural habitat in history. “I had no training, no degree,” says Goodall. “But Louis didn’t care about academic credentials. What he was looking for was someone with an open mind, with a passion for knowledge, with a love of animals, and with monumental patience. My mission was to get close to the chimpanzees, to live among them, to be accepted.”
Written and directed by Brett Morgen based on Goodall’s own writings, with a whimsical orchestral score from composer Philip Glass, Jane gives you a very close insight into Goodall’s research that made connections between chimpanzees and humans (that some tried to discredit), her relationship with Van Lawick (there’s an extremely sweet exchange of proposal via telegram), the results of human interference with animal habitats, and her decision to raise awareness of chimpanzees disappearing across the continent of Africa.
How to watch: Jane is streaming on Disney+.
14. Disney Gallery: The Mandalorian
Credit: Disney
This one is made for people who really love them some Star Wars: a 10-part docuseries about the making of The Mandalorian series. Going behind the scenes of Seasons 1 and 2, Disney Gallery: The Mandalorian examines everything from the special and visual effects to the writing, acting, and directing. You’ll get little tidbits of trivia, like how Star Wars legend Dave Filoni almost didn’t get his first job at Lucasfilm, and that actor Carl Weathers’ character Greef Carga was meant to die in Season 1.
While you’re here and into bounty hunters, check out Under the Helmet: The Legacy of Boba Fett, a comprehensive look at the beloved character, with early costume tests and character development. And there’s more Star Wars ahead…
How to watch: Disney Gallery: The Mandalorian is now streaming on Disney+.
15. Paris to Pittsburgh
Credit: RadicalMedia / National Geographic
If you’re not sure what you can do on a local level to contribute to the goals of the Paris Agreement and aren’t happy to wait around for your federal government to act on climate change, check out this empowering documentary. Directed by National Geographic filmmaker Sidney Beaumont and documentarian Michael Bonfiglio, Paris to Pittsburgh follows Donald Trump’s withdrawal from the Paris Agreement in 2017 and the Mayor of Pittsburgh’s decision for the city to stay in (this was all before Joe Biden’s rejoining in Jan. 2021). Citizens hit the streets, which drove a national movement in cities around America that pledged to uphold the Paris goals and commit to using 100 percent renewable energy.
Narrated by The Marvelous Mrs. Maisel‘s Rachel Brosnahan, the documentary consults climate scientists, geologists, politicians, and local pioneers to understand what those cities are actually doing to achieve this. This includes renewable energy efforts in Adjuntas, Puerto Rico, hammered by extreme weather events like Hurricane Maria, which caused widespread power outages, and Miami, Florida, affected by flooding from rising sea levels. Plus, it names and shames climate deniers within the American government, and points out shameful budget cuts to the EPA by the Trump administration.*
Where to watch: Paris to Pittsburgh is now streaming on Disney+.
16. One Strange Rock + Welcome to Earth
Credit: National Geographic for Disney+ / Kyle Christy
Will Smith teamed up with director Darren Aronofsky for two stunning documentary series that urge you not to take Earth for granted. The earlier of the two, One Strange Rock, is a 10-parter that examines the beautiful weirdness of our planet, as Aronofsky pulls together some truly beautiful footage of Earthly landscapes that look quite alien, or like something you’ve seen in a sci-fi film. In order to take a truly large step back to see the Earth from afar, the documentary series features interviews with astronauts Chris Hadfield, Mae Jemison, Peggy Whitson. Leland Melvin, Mike Massimino, Nicole Stott, Jerry Linenger, and Jeffrey A. Hoffman.
The newer series, Welcome to Earth, sees Smith joining up with explorers around the world to find a new appreciation for how the world connects and moves from a sensory perspective: for example, in the first episode, Smith joins mountaineer Erik Weihenmayer, who is blind, and vulcanologist Jeff Johnson on the edge of a volcano in an effort to understand the spectrum of sound and mapping. In both series, the Hollywood-ness of it all can’t be denied, but the ambitious cinematography, swift pace, and Smith’s seemingly genuine need to understand a new perspective make these both truly engaging series about our planet.
How to watch: One Strange Rock and Welcome to Earth are now streaming on Disney+.
17. Before the Flood
Credit: RatPac Documentary Films
If you’re not into climate change documentaries helmed by celebrities, this one might not be for you. However, even if you’re not a fan of the Hollywood actor, Before the Flood presenter, and U.N. Messenger of Peace Leonardo DiCaprio, his use of star power and sizable budget to draw attention to the climate crisis in this documentary is undeniably impactful, encouraging people to admit what they don’t know and make the decision to get educated. “The truth is, the more I’ve learned about this issue and everything that contributes to the problem, the more I realise how much I don’t know,” he says.
Released in 2016 in conjunction with the Paris Agreement, Before the Flood covers many of the same areas as Al Gore’s films, functioning as a strong primer on climate change, the damage we’ve done, and what’s likely to happen if we fail to act. DiCaprio spent two years traveling to key locations: the melting ice sheets in Kangerlussuaq in Greenland and Baffin Island in the Canadian Arctic; flooding Florida with its electric flood pumps and raised roads; and the Sumatran rainforest, where deforestation is causing wildlife habitat destruction and increased industrial carbon emissions. DiCaprio also interviews a heck of a lot of people, everyone from world leaders, including then-President Barack Obama and Pope Francis, to Arctic explorers and guides, climatologists, astronauts, scholars, economists, marine ecologists, and Elon Musk inside Tesla’s Gigafactory in Nevada.
Though it’s years old now, the arguments are frustratingly the same today.
How to watch: Before the Flood is streaming on Disney+.
18. Fauci
Credit: National Geographic for Disney+ / Visko Hatfield
Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, became a symbol of progress for many Americans during the coronavirus pandemic. Though he’d been in the same job for 40 years, the outbreak of COVID-19 would see Fauci find international fame amid the crisis, and this National Geographic documentary explores this moment amid his broader career. The documentary tracks Fauci through the daily challenges of the pandemic, from NIAID staff meetings to briefings with the president, to personal threats delivered to him and his family by far-right extremists claiming COVID-19 vaccines as part of a hoax. But it also examines Fauci’s incredibly important work during the HIV/AIDS crisis in the ’80s and ’90s, and compares and contrasts this global crisis to our current one.
How to watch: Fauci is streaming on Disney+.
19. Elephant
Credit: Disneynature
One of the collection of Disneynature documentaries on Disney+ more aimed at kids, this sweet film follows a herd of elephants, focusing on 40-year-old Shani and her son Jomo, as they make the eight-month, thousand-mile journey from the Okavango delta across the Kalahari Desert to the Zambezi river. Extremely jovially narrated by Meghan, Duchess of Sussex, the documentary has plenty of beautiful footage of these wondrous creatures, woven together by directors Vanessa Berlowitz and Mark Linfield. The score makes it feel super Disney, and all that footage is just made for an afternoon curled up with the whole family watching the herd roll around in the mud while Meghan announces, “It’s tiiiiiime for a pool party!”
There’s plenty more Disneynature options if you want more like this, with Dolphin Reef and Penguins are worth checking out. And if you want more on the flooding of the Okavango delta in Botswana, check out The Flood, narrated by Angela Bassett, which is also on Disney+.
How to watch: Elephant is now streaming on Disney+.
20. Becoming Cousteau
Credit: The Cousteau Society
A deep dive by definition, Becoming Cousteau explores the depths of the legendary oceanic explorer and filmmaker. Director Liz Garbus delves into the life of Jacques Cousteau, whose documentation of ocean creatures with newly adapted equipment changed the game — all aboard his boat, the Calypso. His work, including 50 books, award-winning films, and long-running television shows, The Undersea World of Jacques Cousteau and The Cousteau Odyssey, was highly influential on aquatic exploration. If you’ve seen Wes Anderson’s The Life Aquatic, you’re gotten a taste of the signature aesthetic. Garbus was granted access to 550 hours of archival material — including over 100 hours of audio journal entries and interviews from collaborators and crew members — to create this documentary, and she brings to life his innovation and dedication to environmentalism, alongside his family life and creation of The Cousteau Society.
How to watch: Becoming Cousteau is now streaming on Disney+.
21. Secrets of the Whales
Credit: National Geographic for Disney+ / Kina Scollay).
When I tell you the cinematography of the National Geographic series Secrets of the Whales is stunning, it’s an understatement 10,000 leagues deep. Filmed over three years, this four-part documentary explores whale culture through orcas, humpbacks, belugas, narwhals, and sperm whales, including looking at how each family speaks a unique language, and how whales experience love, joy, and grief. Directed by Brian Armstrong, the documentary features the National Geographic explorer and renowned whale photographer Brian Skerry, and comes narrated by Sigourney Weaver. This is the kind of documentary you should watch if you’re done with people for a bit, but want to find some kind of human connection with these incredible underwater animals.
How to watch: Secrets of the Whales is now streaming on Disney+.
22. Among the Stars
Credit: NASA / Chris Cassidy
If you really want to know what years in the life of a NASA astronaut can entail, check out Among the Stars, a six-part docuseries that goes behind the scenes of the American space agency on the ground and aboard the International Space Station. Filmed over two years and directed by Ben Turner, the documentary focuses on former NASA astronaut Chris Cassidy — including his final, critical mission aboard the ISS to repair the alpha magnetic spectrometer (AMS) a state-of-the-art particle physics detector. Many other astronauts are featured, including ESA’s Luca Parmitano and NASA’s Drew Morgan. You’ll go inside NASA’s training facilities (including the spacewalk pools), meet the whole team of engineers and specialists involved in the mission, and head all the way to the station, watching video logs, livestreams, and tense helmet cams — especially in the opening spacewalk.
Disney+ has a heap of space documentaries on the platform — check out The Real Right Stuff for the true story behind the film.
How to watch: Among the Stars is now streaming on Disney+.
*Asterisks indicate the writeup is adapted from another article.