Original subtitles

It was a rainy December morning in San Francisco’s Sea Cliff neighborhood

when the Earth opened up.

The ground suddenly fell away into a 12-meter-deep sinkhole

that engulfed a garage, part of a street,

and a luckily unoccupied multi-million-dollar mansion.

Sinkholes can seem to strike out of nowhere,

collapsing roads or even whole city blocks in a flash.

And they’re more common than people often think.

For example, there are tens of thousands of sinkholes

in the US state of Florida alone,

which range from less than a meter to over 100 meters wide.

Sinkholes form when something carves away space underground.

And in nature, that something is usually water.

Rainwater is typically slightly acidic, so it can melt away bedrock,

forming cavities and caves deep below the surface.

At this point, there might be no sign of trouble.

But as water keeps flowing down into the bedrock cavities,

it carries overlying sediment and soil with it,

forming a hidden hole in the soft sediments above.

These sinkholes can collapse without warning,

especially if the uppermost layer concealing the hole is rigid.

In this case, a passing vehicle or pedestrian

might be just enough weight to trigger a collapse.

For example, in West Florida in 2023,

a sinkhole suddenly opened up under a passing car,

trapping its two passengers in a growing pool of muddy water

until they were rescued.

While these types of collapses can be particularly shocking,

many sinkholes are less notable as they develop over longer periods of time.

They can open up gradually as a column of sediment washes down

into a cavity below,

or when water accumulates in a shallow depression of rock

and slowly eats it away.

Some conditions can make sinkholes more likely.

They often form in karst landscapes,

or regions where the bedrock is composed of water-eaten limestone.

This is because limestone is a carbonate rock,

which dissolves easily in acid.

But sinkholes in karst landscapes aren’t always disasters—

some are geological marvels.

The legendary cenotes of Mexico’s Yucatan Peninsula are water-filled sinkholes,

which were used by the ancient Mayans as places of ritual

and were believed to be openings to the underworld.

And sinkholes can also be deep wells of biodiversity.

600 meters down at the bottom of the world’s largest sinkhole,

China’s Xiaozhai Tiankeng, or “Heavenly Pit,

an untouched forest hosts a distinct ecosystem of rare plants and animals,

including ginkgo trees and clouded leopards.

Of course, most sinkholes aren't considered natural wonders.

In fact, many aren't natural at all.

A broken pipe among the mazes of plumbing beneath our cities

can pour water into the ground

and open up an escape channel for sediment-laden water.

A situation like this is what led to the Sea Cliff neighborhood sinkhole in 1995.

More recently, in 2007 in Guatemala City,

the combination of heavy rainstorms and a leaking sewage line

caused a 60-meter-deep sinkhole to open up in the middle of a city street,

taking three victims with it.

And other human activities like pumping up groundwater

and drilling for fossil fuels can also trigger sinkholes.

Meanwhile, climate change is intensifying drought and rainfall around the world,

which together, can carve out sinkholes more effectively.

Storms are obviously powerful agents of erosion.

And droughts can encourage sinkholes to collapse by lowering the water tables,

which help support overlying sediments.

Because many sinkholes are the result of human meddling,

we can prevent them by regulating industry,

limiting the amount of groundwater we pump up,

and by better maintaining plumbing systems.

It's even possible to spot potential sinkholes using tools like radar,

or by running electrical currents through the ground

and measuring its resistance.

But even if we found every sinkhole lurking below the surface today,

there would soon be more to discover.

The Earth might seem like it’s standing still,

but there’s a lot going on just below your feet.