In 1796, Thomas Jefferson received a box of bones he couldn't identify.
A long, sharp claw reminded him of a lion, but the arm bones suggested a larger animal, one about three meters long.
Thinking it might be huge unknown species of North American lion, Jefferson warned explorers Lewis and Clark to keep an eye out for this mysterious predator.
But Jefferson's box of bones didn't come from a lion.
They came from an extinct giant sloth.
Prehistoric ground sloths first appeared around 35 million years ago.
Dozens of species lived across North, Central and South America, alongside other ancient creatures like mastodons and giant armadillos.
Some ground sloths, like the megalonychid, were cat-sized, but many were massive.
Jefferson's sloth, Megalonyx, weighed about a ton, and that was small compared to megatherium, which could reach six metric tons, as much as an elephant.
They ambled through the forests and savannas using their strong arms and sharp claws to uproot plants and climb trees, grazing on grasses, leaves, and prehistoric avocados.
In fact, we might not have avocados today if not for the giant sloths.
Smaller animals couldn't swallow the avocado's huge seed, but the sloths could, and they spread avocado trees far and wide.
Ground sloths flourished for millions of years, but around 10,000 years ago, they started disappearing along with the Western Hemisphere's other giant mammals.
Researchers think that ground sloths could have been pushed out by an oncoming ice age, or competition with other species, maybe humans, who arrived in the region around the time most of the sloths went extinct.
Some of the smaller sloths did survive and migrated to the treetops.
Today, there are six species left living in the rainforest canopies of Central and South America.
Hanging out in the trees is a good way to avoid predators, and there are plenty of leaves to eat.
But this diet has its drawbacks.
Animals extract energy from food and use that energy to move around, maintain their body temperature, keep their organs working, and all the other activities necessary for survival.
But leaves don't contain much energy, and that which they do have is tough to extract.
Most herbivores supplement a leafy diet with higher energy foods like fruit and seeds.
But sloths, especially three-toed sloths, rely on leaves almost exclusively.
They've evolved finely tuned strategies for coping with this restricted diet.
First, they extract as much energy from their food as possible.
Sloths have a multi-chambered stomach that takes up a third of their body, and depending on the species, they can spend five to seven days, or even weeks, processing a meal.
The other piece of the puzzle is to use as little energy as possible.
One way sloths do this is, of course, by not moving very much.
They spend most of their time eating, resting, or sleeping.
They descend from the canopy just once a week for a bathroom break.
When sloths do move, it's not very fast.
It would take a sloth about five minutes to cross an average neighborhood street.
This unhurried approach to life means that sloths don't need very much muscle.
In fact, they have about 30% less muscle mass than other animals their size.
Sloths also use less energy to keep themselves warm because their body temperature can fluctuate by about five degrees Celsius, less than a cold-blooded reptile, but more than most mammals.
These physical and behavioral adaptations minimize the sloth's energy expenditure, or metabolic rate.
Three-toed sloths have the slowest metabolism of any mammal.
The giant panda is second slowest, and two-toed sloths come in third.
Moving slowly has allowed sloths to thrive in their treetop habitat.
But it's also made the sloths themselves a great habitat for other organisms, including algae, which provides a little extra camouflage, and maybe even a snack.
Sloths may not be giant anymore, but that doesn't make them any less remarkable.
I was raised in Seoul, Korea, and moved to New York City in 1999 to attend college.
I was pre-med at the time, and I thought I would become a surgeon because I was interested in anatomy and dissecting animals really piqued my curiosity.
At the same time, I fell in love with New York City.
I started to realize that I could look at the whole city as a living organism.
I wanted to dissect it and look into its unseen layers.
And the way to it, for me, was through artistic means.
So, eventually I decided to pursue an MFA instead of an M.D.
and in grad school I became interested in creatures that dwell in the hidden corners of the city.
In New York City, rats are part of commuters' daily lives.
Most people ignore them or are frightened of them.
But I took a liking to them because they dwell on the fringes of society.
And even though they're used in labs to promote human lives, they're also considered pests.
I also started looking around in the city and trying to photograph them.
One day, in the subway, I was snapping pictures of the tracks hoping to catch a rat or two, and a man came up to me and said, "You can't take photographs here.
The MTA will confiscate your camera."
I was quite shocked by that, and thought to myself, "Well, OK then.
I'll follow the rats."
Then I started going into the tunnels, which made me realize that there's a whole new dimension to the city that I never saw before and most people don't get to see.
Around the same time, I met like-minded individuals who call themselves urban explorers, adventurers, spelunkers, guerrilla historians, etc.
I was welcomed into this loose, Internet-based network of people who regularly explore urban ruins such as abandoned subway stations, tunnels, sewers, aqueducts, factories, hospitals, shipyards and so on.
When I took photographs in these locations, I felt there was something missing in the pictures.
Simply documenting these soon-to-be-demolished structures wasn't enough for me.
So I wanted to create a fictional character or an animal that dwells in these underground spaces, and the simplest way to do it, at the time, was to model myself.
I decided against clothing because I wanted the figure to be without any cultural implications or time-specific elements.
I wanted a simple way to represent a living body inhabiting these decaying, derelict spaces.
This was taken in the Riviera Sugar Factory in Red Hook, Brooklyn.
It's now an empty, six-acre lot waiting for a shopping mall right across from the new Ikea.
I was very fond of this space because it's the first massive industrial complex I found on my own that is abandoned.
When I first went in, I was scared, because I heard dogs barking and I thought they were guard dogs.
But they happened to be wild dogs living there, and it was right by the water, so there were swans and ducks swimming around and trees growing everywhere and bees nesting in the sugar barrels.
The nature had really reclaimed the whole complex.
And, in a way, I wanted the human figure in the picture to become a part of that nature.
When I got comfortable in the space, it also felt like a big playground.
I would climb up the tanks and hop across exposed beams as if I went back in time and became a child again.
This was taken in the old Croton Aqueduct, which supplied fresh water to New York City for the first time.
The construction began in 1837.
It lasted about five years.
It got abandoned when the new Croton Aqueducts opened in 1890.
When you go into spaces like this, you're directly accessing the past, because they sit untouched for decades.
I love feeling the aura of a space that has so much history.
Instead of looking at reproductions of it at home, you're actually feeling the hand-laid bricks and shimmying up and down narrow cracks and getting wet and muddy and walking in a dark tunnel with a flashlight.
This is a tunnel underneath Riverside Park.
It was built in the 1930s by Robert Moses.
The murals were done by a graffiti artist to commemorate the hundreds of homeless people that got relocated from the tunnel in 1991 when the tunnel reopened for trains.
Walking in this tunnel is very peaceful.
There's nobody around you, and you hear the kids playing in the park above you, completely unaware of what's underneath.
When I was going out a lot to these places, I was feeling a lot of anxiety and isolation because I was in a solitary phase in my life, and I decided to title my series "Naked City Spleen," which references Charles Baudelaire.
"Naked City" is a nickname for New York, and "Spleen" embodies the melancholia and inertia that come from feeling alienated in an urban environment.
This is the same tunnel.
You see the sunbeams coming from the ventilation ducts and the train approaching.
This is a tunnel that's abandoned in Hell's Kitchen.
I was there alone, setting up, and a homeless man approached.
I was basically intruding in his living space.
I was really frightened at first, but I calmly explained to him that I was working on an art project and he didn't seem to mind and so I went ahead and put my camera on self-timer and ran back and forth.
And when I was done, he actually offered me his shirt to wipe off my feet and kindly walked me out.
It must have been a very unusual day for him.
(Laughter) One thing that struck me, after this incident, was that a space like that holds so many deleted memories of the city.
That homeless man, to me, really represented an element of the unconscious of the city.
He told me that he was abused above ground and was once in Riker's Island, and at last he found peace and quiet in that space.
The tunnel was once built for the prosperity of the city, but is now a sanctuary for outcasts, who are completely forgotten in the average urban dweller's everyday life.
This is underneath my alma mater, Columbia University.
The tunnels are famous for having been used during the development of the Manhattan Project.
This particular tunnel is interesting because it shows the original foundations of Bloomingdale Insane Asylum, which was demolished in 1890 when Columbia moved in.
This is the New York City Farm Colony, which was a poorhouse in Staten Island from the 1890s to the 1930s.
Most of my photos are set in places that have been abandoned for decades, but this is an exception.
This children's hospital was closed in 1997; it's located in Newark.
When I was there three years ago, the windows were broken and the walls were peeling, but everything was left there as it was.
You see the autopsy table, morgue trays, x-ray machines and even used utensils, which you see on the autopsy table.
After exploring recently-abandoned buildings, I felt that everything could fall into ruins very fast: your home, your office, a shopping mall, a church -- any man-made structures around you.
I was reminded of how fragile our sense of security is and how vulnerable people truly are.
I love to travel, and Berlin has become one of my favorite cities.
It's full of history, and also full of underground bunkers and ruins from the war.
This was taken under a homeless asylum built in 1885 to house 1,100 people.
I saw the structure while I was on the train, and I got off at the next station and met people there that gave me access to their catacomb-like basement, which was used for ammunition storage during the war and also, at some point, to hide groups of Jewish refugees.
This is the actual catacombs in Paris.
I explored there extensively in the off-limits areas and fell in love right away.
There are more than 185 miles of tunnels, and only about a mile is open to the public as a museum.
The first tunnels date back to 60 B.C.
They were consistently dug as limestone quarries and by the 18th century, the caving-in of some of these quarries posed safety threats, so the government ordered reinforcing of the existing quarries and dug new observation tunnels in order to monitor and map the whole place.
As you can see, the system is very complex and vast.
It's very dangerous to get lost in there.
And at the same time, there was a problem in the city with overflowing cemeteries.
So the bones were moved from the cemeteries into the quarries, making them into the catacombs.
The remains of over six million people are housed in there, some over 1,300 years old.
This was taken under the Montparnasse Cemetery where most of the ossuaries are located.
There are also phone cables that were used in the '50s and many bunkers from the World War II era.
This is a German bunker.
Nearby there's a French bunker, and the whole tunnel system is so complex that the two parties never met.
The tunnels are famous for having been used by the Resistance, which Victor Hugo wrote about in "Les Miserables."
And I saw a lot of graffiti from the 1800s, like this one.
After exploring the underground of Paris, I decided to climb up, and I climbed a Gothic monument that's right in the middle of Paris.
This is the Tower of Saint Jacques.
It was built in the early 1500s.
I don't recommend sitting on a gargoyle in the middle of January, naked.
It was not very comfortable.
(Laughter) And all this time, I never saw a single rat in any of these places, until recently, when I was in the London sewers.
This was probably the toughest place to explore.
I had to wear a gas mask because of the toxic fumes -- I guess, except for in this picture.
And when the tides of waste matter come in it sounds as if a whole storm is approaching you.
This is a still from a film I worked on recently, called "Blind Door."
I've become more interested in capturing movement and texture.
And the 16mm black-and-white film gave a different feel to it.
And this is the first theater project I worked on.
I adapted and produced "A Dream Play" by August Strindberg.
It was performed last September one time only in the Atlantic Avenue tunnel in Brooklyn, which is considered to be the oldest underground train tunnel in the world, built in 1844.
I've been leaning towards more collaborative projects like these, lately.
But whenever I get a chance I still work on my series.
The last place I visited was the Mayan ruins of Copan, Honduras.
This was taken inside an archaeological tunnel in the main temple.
I like doing more than just exploring these spaces.
I feel an obligation to animate and humanize these spaces continually in order to preserve their memories in a creative way -- before they're lost forever.
Thank you.
So 24 years ago, I was brought to The New Yorker as art editor to rejuvenate what had by then become a somewhat staid institution and to bring in new artists and to try to bring the magazine from its ivory tower into engaging with its time.
And it was just the right thing for me to do because I've always been captivated by how an image can -- a simple drawing -- can cut through the torrent of images that we see every single day.
How it can capture a moment, how it can crystallize a social trend or a complex event in a way that a lot of words wouldn't be able to do -- and reduce it to its essence and turn it into a cartoon.
So I went to the library and I looked at the first cover drawn by Rea Irvin in 1925 -- a dandy looking at a butterfly through his monocle, and we call it Eustace Tilley.
And I realized that as the magazine had become known for its in-depth research and long reports, some of the humor had gotten lost along the way, because now often Eustace Tilley was seen as a haughty dandy, but in fact, in 1925, when Rea Irvin first drew this image, he did it as part of a humor magazine to amuse the youth of the era, which was the flappers of the roaring twenties.
And in the library, I found the images that really captured the zeitgeist of the Great Depression.
And it showed us not just how people dressed or what their cars looked like, but also what made them laugh, what their prejudices were.
And you really got a sense of what it felt like to be alive in the '30s.
So I called on contemporary artists, such as Adrian Tomine here.
I often call on narrative artists -- cartoonists, children's book authors -- and I give them themes such as, you know, what it's like to be in the subway, or Valentine's Day, and they send me sketches.
And once the sketches are approved by the editor, David Remnick, it's a go.
And I love the way those images are actually not telling you what to think.
But they do make you think, because the artist is actually -- it's almost a puzzle; the artist is drawing the dots, and you, the reader, have to complete the picture.
So to get this image on the left by Anita Kunz, or the one on right by Tomer Hanuka, you have to play spot the differences.
And it is something that ...
It's really exciting to see how the engagement with the reader ...
how those images really capture -- play with the stereotypes.
But when you get it, it rearranges the stereotypes that are in your head.
But the images don't just have to show people, sometimes it can be a feeling.
Right after September 11, I was at a point, like everybody else, where I really didn't know how to deal with what we were going through, and I felt that no image could capture this moment, and I wanted to just do a black cover, like no cover.
And I talked to my husband, cartoonist Art Spiegelman, and mentioned to him that I was going to propose that, and he said, "Oh, if you're going to do a black cover, then why don't you do the silhouette of the Twin Towers, black on black?"
And I sat down to draw this, and as soon as I saw it, a shiver ran down my spine and I realized that in this refusal to make an image, we had found a way to capture loss and mourning and absence.
And it's been a profound thing that I learned in the process -- that sometimes some of the images that say the most do it with the most spare means.
And a simple image can speak volumes.
So this is the image that we published by Bob Staake right after the election of Barack Obama, and captured a historic moment.
But we can't really plan for this, because in order to do this, we have to let the artist experience the emotions that we all feel when that is happening.
So back in November 2016, during the election last year, the only image that we could publish was this, which was on the stand on the week that everybody voted.
(Laughter) Because we knew somebody would feel this -- (Laughter) when the result of the election was announced.
And when we found out the result, we really were at a loss, and this is the image that was sent by Bob Staake again, and that really hit a chord.
And again, we can't really figure out what's going to come next, but here it felt like we didn't know how to move forward, but we did move forward, and this is the image that we published after Donald Trump's election and at the time of the Women's March all over the US.
So over those 24 years, I have seen over 1,000 images come to life week after week, and I'm often asked which one is my favorite, but I can't pick one because what I'm most proud of is how different every image is, one from the other.
And that's due to the talent and the diversity of all of the artists that contribute.
And now, well, now, we're owned by Russia, so -- (Laughter) In a rendering by Barry Blitt here, Eustace has become Eustace Vladimirovich Tilley.
And the butterfly is none other than a flabbergasted Donald Trump flapping his wings, trying to figure out how to control the butterfly effect, and the famed logo that was drawn by Rae Irvin in 1925 is now in Cyrillic.
So, what makes me really excited about this moment is the way that ...
You know, free press is essential to our democracy.
And we can see from the sublime to the ridiculous that artists can capture what is going on in a way that an artist armed with just India ink and watercolor can capture and enter into the cultural dialogue.
It puts those artists at the center of that culture, and that's exactly where I think they should be.
Because the main thing we need right now is a good cartoon.
Thank you.
(Applause)