How do sinkholes swallow entire houses and cities? - Elise Cutts

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.