It’s hard to believe that it wasn’t all a nightmare.

Around 11 on Thursday night, Jeffrey Bush, 37, was tucked in his suburban Tampa bed when, without warning, his entire bedroom collapsed, swallowed up by a 30-foot-wide, 20-foot-deep sinkhole. His brother, Jeremy, rushed in and tried to help, but ended up being pulled out by police as the hole collapsed.

“All I could see was the top of his bed,” said Bush. “So I jumped in the hole and tried digging him out. I thought I could hear him screaming for me and hollering for me, but they couldn’t do nothing.”

Rescue teams lowered a microphone and video equipment into the hole but found no signs of life. As of Friday afternoon, Jeffrey is presumed dead.

Subsidances aren’t rare in central Florida or around the world, but a sinkhole causing a human death definitely is. Read on for The Daily Beast’s guide to spotting potential sinkholes and understanding them.

What causes sinkholes?

Naturally occurring sinkholes, like the one that killed Jeremy Bush, are depressions in the earth caused by water erosion of the bedrock below a land surface. Acidic rainwater seeps through the ground, reaches soluble bedrock (usually salt, sandstone, or a carbonate rock such as limestone), dissolves small amounts, and carries the particles away. Over time (even thousands of years), this process can enlarge natural pores and cracks in the bedrock, to the point where large cavities or caves are formed. And with a gaping hole underground, there’s nothing to support the weight of layers of sediment above—that's when the ground collapses.

As The Atlantic points out, long periods of drought followed by rain can also set off collapses. Droughts cause groundwater tables to drop, and caves that were once filled with water and were therefore supported by it become weaker. Once rain finally comes, the extra weight of the soaked-through top layer of earth can cause the cave to collapse. And beware of tapping into groundwater for agriculture too, as that can have the same effect as a drought.

Non-naturally occurring sinkholes can form because of water main breaks, sewer collapses, or even abandoned mines. If there’s a substantial change in the weight of a land surface, such as when industrial or runoff-storage ponds are created, underground collapses may also be triggered.

Where are they most likely to occur?

Sinkholes are a worldwide phenomenon—geologists estimate that 10 percent of Earth’s surface (including the entirety of Florida) is shaped by dissolving bedrock prone to sinkholes, a type of landscape called karst topography. Nearly every U.S. state is covered at least in part by karst topography and sinkholes are considered most common in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. Check out the U.S. Geological Survey’s map of the U.S.’s karst areas here.

How big do these things get?

Pretty massive, as it turns out. The Qattara Depression in Egypt extends to 440 feet below sea level, and a length of 186 miles (with a width of 95 miles). The 2007 and 2010 sinkholes in Guatemala City were 330 feet and 30 stories deep, respectively. And in Sarisarinama, Venezuela, multiple sinkholes have reached about 1,000 feet wide and 1,150 feet deep. Of course, sinkholes aren’t always gargantuan; they can also be quite small, no more than 2 or 3 feet in diameter and depth.

Are sinkhole fatalities common?

No, which is what makes the Florida case so bizarre. USA Today talked to Anthony Randazzo, a former University of Florida professor and contractor who has spent his career studying sinkholes. Randazzo recalled only two other people in 40 years who have died because of them—and even then, it was because both people had been drilling water wells. (Remember what we said above about tapping into groundwater?!) It’s worth noting that both these deaths also occurred in Florida.

How can I tell if I’m living on a sinkhole and how can I fix it?

Consider sagging trees or fence posts, doors or windows that won’t close properly, and rainwater collecting in unusual spots as warning signs. Get the hell outta there and, if it’s on public property, report it to local law enforcement. If it’s on your own property, a small hole can be filled with natural materials like rock and clayey sand. Larger sinkholes will require the help of experts. Professional geologists or a geotechnical engineering firm can help by injecting grout into the area to fill up cracks and strengthen the foundation.