This is 99% invisible. I'm Roman Mars.

When you're on a highway stuck in bumper to bumper traffic,

all cities start to feel the same.

It's easy to forget that you're in a particular place on the Earth's surface, a coast or a bay or a desert or a plane.

Mexico City is one of the biggest, busiest cities in the world.

But it's also a Highland Valley, over 7,000 feet

in the air, surrounded on all sides by volcanoes.

Sometimes these volcanoes split up a little fire and smoke to remind people of the volatile geology all around them.

But about a millennium and a half ago, a volcano called Sheetlay, fully erupted.

Molten lava poured into the valley destroying an indigenous city and covering 30 square

miles in a bed of volcanic stone. Today most of this lava field has been

paved over with roads and buildings and parking lots devoured by the largest city in North America, Mexico City.

That's producer Emmett Fitzgerald.

But in the midst of all this urban sprawl, there's one spot where you can still get a good look at that old volcanic rock.

So you can see now in 2019 rocks that are exactly the same as 1,600 years ago.

This is Pedro Camarina.

He's a landscape architect,

and we're in the Pedro Gal de Sangell Ecological Reserve.

It's a small park in South Mexico City

that contains the last remnants of the old lava field.

Looking down at the ground, you can see swirls and folds in the rock, how the lava was moving as it

cooled all those centuries ago.

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