Last week, an enormous sinkhole opened up in the middle of a road in Fukuoka, Japan. The sinkhole, which measured nearly 90 feet wide, 100 feet long and 50 feet deep, may have been caused by construction work on a nearby subway line. Fortunately, no one appears to have been seriously injured.

Sinkholes are not usually as spectacular as the one in Fukuoka, but they can cause a lot of damage when they appear. Here's what you need to know about what causes sinkholes, where they form, and why you shouldn't fill the one in your backyard with garbage.

What are they?

When geologists refer to sinkholes, they're usually thinking of holes that form naturally, rather than because of human interference (like subway construction). "The term is often used as sort of a catchall for things that are just ground collapses, but not all ground collapses are in geologic terms true sinkholes," says Dan Doctor, a research geologist with the U.S. Geological Survey.