Around 1,500 years ago, massive pyramids stood in the foothills of the Andes  Mountains.

Casarabe farming families moved through a web of streets,  plazas, canals, and buildings. Causeways connected dozens of urban settlements.

And then, around 600 years ago, they all vanished. We don’t know why. Over time, a rainforest sprang up  over the abandoned settlements.

Trees and vines filled the canals  and took over the pyramids. The rainforest flourished as if the whole civilization had never existed at all. It’s hard to even imagine: a millennium-old civilization five times the size  of New York City just… gone.

But the Earth is amazingly  good at wiping the slate clean. And one day, it is likely to do  the same with our modern world. Because one day, all humans  will almost certainly be gone… whether we do ourselves in or die out in a plague, or a meteor strike, or some other catastrophe.

And when that happens, whatever that is, our bodies, artifacts, and cities  will all return to the Earth. Forests, fire, water, and microbes will overtake and break down almost every trace of us. Our planet will move on, as it always has.

But some things we have made and  altered will last much longer than others, and they’re probably  not the things you’d expect. So let’s take a trip to the distant future and see what humanity will leave behind. [♪INTRO] It’s hard to look at a city like New  York, with its towering buildings and all that concrete and steel,  and imagine it ever disappearing. Modern cities just seem more  permanent than a web of settlements in the pre-Columbian Amazon, no  matter how advanced they were.

But if people suddenly  abandoned New York one day — or Tokyo or Nairobi or any other city — it wouldn’t be long before nature rushed back in. Just look at what has happened to the area around Chernobyl in just 40 years. In major cities, water would  penetrate and destroy buildings, plants would pop up through roads,  steel bridges would corrode, and sooner or later, wood structures would burn.

Before long, forests and marshes would return. If an alien spacecraft flew  over Earth thousands of years after we’re gone, they’d stare down  at a seemingly pristine natural landscape, with few obvious  signs that we were ever here. But that natural landscape wouldn’t  be quite as pristine as it looked.

Because even if the cities themselves are gone, any place we once transformed can  never quite be the same again. We humans have a habit of showing up somewhere beautiful and leaving our mark. Take the Canary Islands.

When the  first people arrived more than 2,000 years ago, they found a  volcanic landscape covered in trees and then they started clearing them for farming. Later on, nature returned,  but not the original forest. Even today, the island is  full of shrubs and herbs, but the original forest never bounced back.

When humans upset the balance of an ecosystem, nature usually picks itself  up and finds a new balance, instead of returning to how it used to be. That’s what has happened over the last few decades in lakes all over North America, ever since people accidentally  introduced zebra mussels. Zebra mussels devour the plankton  that other lake life depends on.

Without enough food, the numbers of native mussels and clams have plummeted. Meanwhile, all the zebra mussels’  waste falls to the lakebed, so the leeches and crustaceans  down there are having a feast, along with a bunch of bacteria. At this point, the flow and balance of these ecosystems has completely shifted.

Even if we could completely  get rid of the zebra mussels, those ecosystems would never be the same. Thousands of years down the line,  our fingerprint will still be there. It might not point clearly back to  humans or tell alien explorers how clumsy we were, but it’s a mark  we’ve made that will always be there.

Of course, some evidence of our time  here will be a lot more concrete. And I’m not talking about concrete itself, because most of that stuff will  crumble within a few centuries! I’m talking about things  like glassware and ceramics.

The same kinds of objects archaeologists typically dig up at the sites  of ancient civilizations. These things last basically forever, thanks to their chemical  composition in other words, the way they’re stuck together. In glass, silicon and oxygen atoms lock together with extremely strong bonds.

The way they’re arranged, it’s really hard for any other chemicals to tear them apart. That’s why natural forms of  glass, like obsidian and basalt formed in volcanoes, can be millions of years old. Ceramics work differently, but  they’re really strong as well.

In these materials, non-metal atoms  steal electrons from metal atoms. This creates opposite charges  that stick together like magnets. Even though both glass and  ceramic shatter very easily, they never really break down chemically.

Long after we’re gone, broken jars and bathroom tiles will still litter  the ground of former cities. Millions of years from now, future  species or extraterrestrial visitors wading through forests might  pick them up and wonder about the species that made all  this incredibly durable junk. These random shards wouldn’t say  a whole lot about who we were, let alone hint at what kind of  cities once stood in those spots.

But future explorers could tell that some kind of technologically advanced society was here before. Weirdly, though, some of the  most long-lasting traces of human civilization might not  be from modern humanity at all. They might be the remains  of much older civilizations.

Around 10 meters beneath Naples,  there’s an entire buried city, complete with roads, buildings,  aqueducts, and burial chambers. It was built by the ancient  Greeks around 2,500 years ago. Archaeologists can’t access very much of it, because there’s a modern  city parked on top of it — they can’t just start digging.

But over the years, people  have accidentally dug into it while building things like  cisterns and bomb shelters. Which, I mean, imagine trying  to build a bomb shelter at the height of World War II and  finding a 2500-year-old city. And even though this ancient  city is about a millennium older than the Casarabe settlements  buried under the Amazon, It is incredibly well preserved.

Some ancient frescoes and  sculptures are still intact. Which goes to show that a city that’s  been protected from the elements can stick it out much longer than  a city that’s constantly facing things like rain, wind, snow,  fire, and swinging temperatures. So, ancient buried cities like this one could outlast many of our modern ones.

That means that future explorers  might completely skip over modern society and just assume  humanity peaked in BCE times. Either way, imagine trying to  grasp what we were all about based on ancient underground ruins and some random objects like glass shards and broken ceramics. That might not be all they  have to work with though.

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To learn more or register  for the next upcoming test, scan the QR code or click  the link in the description. Today, one of the ways we can  tell where ancient humans lived, even if there’s no obvious trace of them, is through their mark on the planet itself. In huge stretches of Amazon, there are patches of soil that are darker than  the soil surrounding them.

They’re called terra preta,  meaning “dark earth,” and they get their color from high levels of  an organic material called char. Char is a kind of charcoal that you get by burning things like wood, plants, or manure. It's totally natural, but these patches of rainforest didn’t get this way naturally.

Around 9,000 years ago, it looks like people started spreading this  around patches of the Amazon, most likely to make the soil more fertile. They’ve been gone for around a thousand years, but this trace of them is still there. These days, we’re changing the soil  a lot too, all over the planet.

Ever since the 1950s, soil  everywhere has contained little bits of debris known as  spheroidal carbonaceous particles. And unlike char, these  particles don’t form in nature. We make them when we burn coal and oil at temperatures higher than 1000 degrees Celsius.

If future explorers analyze  soil layers the that way we do, with ice or soil cores,  they’ll find a weird spike in these weird particles dating back to our era. And even if they have no idea  what a human was, they will know that something artificial suddenly  transformed the environment. They might guess that an industrialized species had come before them.

And they might even be able to create a map of our major modern societies,  using the concentration of particles to figure out  where we were clustered. Now, all these clues could  reveal a lot about humanity, but in a way, they miss the essence of us. I mean, our stories, our music, our art, all these intangible things that make us human most of these will vanish with us.

Maybe some artwork will survive  millennia in underground or buried cities, like the  ancient frescoes under Naples. But most of our art is not tucked away for safekeeping at least not on these time scales. There is one type of art, though, that  could survive almost indefinitely.

The bottom of the Mediterranean  Sea is littered with shipwrecks dating to ancient times in  particular, back to the Bronze Age. This was when people realized  that they could mix copper with a small amount of tin  to create a tougher metal. Bronze is ridiculously durable, and  there are a few reasons for that.

Metals like iron rust because they combine with oxygen and water around them to form that flaky brown stuff you see  on old bridges and railings. But bronze has a different chemical makeup. When the copper in bronze combines  with oxygen in the environment, instead of rusting, it forms a  tough outer layer called a patina.

It’s basically an armor that protects  it from corroding any further. The tin in there helps too. It also forms a stable outer layer that protects it from being attacked by water.

More than 5,000 years ago, people began using bronze to make weapons, art, and other goods that they ferried around the Mediterranean. Some of them ended up at the bottom of the sea Which was terrible for the people at the time, but for the people of the  future, it’s still there. In 1902, divers pulled up a full statue of a Greek youth dating back to the fourth century BCE still in great shape after  thousands of years underwater.

So long after our books and  instruments and paintings are gone, future species exploring this planet  might come across our bronze art. And they’d be able to see what we looked like, and what we cared about.… Or maybe they’ll dig up ancient weapons and wonder what kind of  species made both beautiful art and tools for destroying each  other out of this same stuff. No matter what, our planet will  reshape itself after we’re gone.

Given enough time, even the  ceramics and glass and underground cities will become part of its  ambitious recycling program. But it’s nice to know some things will definitely survive long after our chapter is over. I’m starting to think we should  make a bronze SciShow keychain.

That would be the kind of thing  we should have thought of, like a couple of weeks ago, but we didn’t! [♪OUTRO]