Authorities fear a grandmother in western Pennsylvania who disappeared while looking for her cat may have been swallowed by a sinkhole.

Crews lowered a pole camera with a sensitive listening device into the hole on Tuesday but no sound was detected, while a second camera lowered down showed what could be a shoe.

Police say Elizabeth Pollard’s relatives called police at about 1 a.m. to say she hadn’t been seen since Monday evening when she went to search for her cat. They found Pollard’s 5-year-old granddaughter in her parked car near the manhole-sized opening.

Here are some things to know about sinkholes:

What are sinkholes?

A sinkhole is an area of ground that has no natural external surface drainage and can form when the ground below the land surface can no longer support the land above, according to the U.S. Geological Survey.

The land usually stays intact for a period of time until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden, dramatic collapse of the land surface can happen.

How common are sinkholes?

Sinkholes are most common in what geologists call karst terrain, which involves types of rock including limestone below the land surface that can naturally be dissolved by groundwater circulating through them. They can also happen due to old underground mines.

The most damage from sinkholes in the U.S. tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. Florida, for example, is highly susceptible to sinkholes because it sits above limestone.

How big are sinkholes?

Sinkholes can range in size from holes that are just a few feet wide to ones that cover a vast area spanning hundreds of acres. Their depth can also vary from just a few inches to more than 100 feet (more than 30 meters). Some are shaped like shallow bowls or saucers, whereas others have vertical walls. Some hold water and form ponds.

Other recent sinkholes

In June, a giant sinkhole in southern Illinois swallowed the center of a soccer field built on top of a limestone mine, taking down a large light pole and leaving a gaping chasm where squads of kids often play. No one was hurt.

In 2023, a sinkhole that in 2013 fatally swallowed a man sleeping in his house in suburban Tampa, Florida, reopened for a third time, but it was behind chain-link fencing and caused no harm to people or property. Officials said the sinkhole reopening was not unusual, especially in central Florida with its porous limestone base.

A large sinkhole opened up in 2020 in South Dakota near where a man was mowing his lawn. Testing revealed a large, improperly sealed mine beneath part of the housing subdivision, and a 40-foot-deep (12-meter-deep) pit mine in another corner of the neighborhood, a lawyer for some of the area homeowners said. Since the first giant collapse, more sinkholes have appeared.