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 multimillion-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.
