Definition
A sinkhole is what happens when underground rock dissolves or soil washes away, leaving a big empty space, and the ground above can't hold its own weight and suddenly caves in.
Giant Holes Made by Nature
Picture a sandcastle you packed tight on the beach. If seawater seeps in underneath, it still looks fine, but the moment you step on it, it crumbles. Natural sinkholes form in a very similar way.
They most often happen when limestone dissolves in water and leaves a huge empty space underground. Rainwater picks up carbon dioxide from the air and becomes weakly acidic. When this acidic rain soaks into the ground, it slowly eats away at limestone, which is made of calcium carbonate, over tens of thousands of years. This process is called dissolution.
Over time, caves of all sizes form underground. If a cave keeps growing, or the layer of soil and rock above it gets thinner, the ceiling can no longer hold up the weight of the surface. It collapses in an instant, and a deep pit appears.
Why Asphalt Collapses in the Middle of a City
Sinkholes often appear in the middle of cities too, where there's no natural limestone and everything is covered in concrete. City sinkholes are caused less by natural erosion and more by underground construction and aging infrastructure.
When an old water or sewer pipe buried underground cracks, the water that gushes out slowly carves away the soil around it. Along the water's path, groundwater and soil get washed away together, and a large empty space forms right under the asphalt. The road looks normal on the surface, but underneath it's like a hollow eggshell.
Then a heavy truck or bus rolls over it. The thin asphalt shell snaps, and the car and the road drop straight down. Sinkholes can also happen when too much groundwater is pumped out during big subway projects or when laying the foundations of tall buildings, because the surrounding ground sinks.
A Bit More Precisely
A sinkhole looks like it appears in the blink of an eye. But usually it's the end result of an empty space that has been growing underground for a while. In limestone regions, that can take thousands of years. With a leaking city pipe, it can take anywhere from a few weeks to a few years.
Normally, groundwater fills the gaps between soil and sand and the caves underground, and it helps hold up some of the weight of the ground above. The pressure created by the water between soil grains is called pore water pressure. But when a drought hits, or a lot of groundwater is pumped out and the water drains away, the support that the groundwater provided disappears. The full weight of the soil presses down, and the ceiling of the empty space collapses more easily.
So a sinkhole doesn't always come with no warning at all. Signs may show up first, like a road that gets slightly bumpy or gaps opening between paving stones. Shallow voids just under a road can sometimes be found ahead of time with equipment such as ground-penetrating radar (GPR).
Common misconceptions
A sinkhole is the ground tearing or splitting apart, like in an earthquake.
A sinkhole isn't the ground tearing from movement of the Earth's crust. Groundwater or rainwater dissolves or washes away soil and rock underground, and the empty space that forms can't support the weight of the surface, so it collapses.
Where you meet it
In limestone regions, rainwater can dissolve the rock and reveal a huge emerald-colored pool underground. This is a natural sinkhole called a cenote.
Under a city street, an old water main bursts and washes soil away, and the asphalt road suddenly caves in. This is a human-made sinkhole accident.
In one sentence
A sinkhole is the sudden collapse of the ground's surface when it can't hold up over an underground space that has dissolved or washed away.