A Hidden Cause of Sinkholes Is Lurking Underground

- [Narrator] An active mine in Illinois suddenly collapsed, causing this 100-foot-wide sinkhole to swallow up part of a soccer field, lights and all.

Sinkholes can happen just about anywhere: on roads, under homes, and sometimes in remote areas.

But in the U.S., they happen most often in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania, anywhere the bedrock is vulnerable to erosion by groundwater.

In many cases, the main trigger for sinkhole is water, either too much of it or not enough.

If groundwater dries up, like during a drought, it can take away support for the ground above, causing it to sink.

But if there's too much water, like during big storms or water main breaks, the sudden influx of groundwater can also cause instability.

The kind of sinkhole that opened up on the Illinois soccer field is actually kind of unusual in that it was spurred on by an active mine, but it's not unprecedented.

Near Carlsbad, New Mexico, giant sinkholes from mining activities started to pop up in 2008.

Then officials discovered that one well that was being mined near the city was on the brink of collapse.

Years of extracting the salt created a wide underground cavity.

A sinkhole seemed imminent, and if it collapsed, it would've taken part of the city with it.

What happened next was a long and expensive remediation effort, and today that brine well is filled.

Sinkholes can form either really quickly or really slowly over a long period of time, and they don't always come with warning signs.

Limestone mines, like the one in Illinois, are usually carefully engineered to prevent collapse.

Pillars are often carved out to hold up the ground above, but when mines do collapse and cause sinkholes, here's what's usually happening underground.

Underground mines are usually deep in bedrock, and over time, cracks can form in the bedrock roof over the mine, and water and sediment slowly trickle down into the mine, leaving behind a new cavity in the rock or soil layer above.

Eventually, when the top layer of soil can no longer support weight, it gives way, forming a dramatic hole in the ground, a sinkhole.

And sinkholes are usually round because the shape of a circle takes the least amount of energy to create.

Think about how when you dig a hole in the sand at the beach the surrounding sand uniformly falls in around it, making a circle.

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