Last November, the ground beneath a busy city street in Fukuoka, Japan, suddenly collapsed. It left a 30 meter (98 foot)-wide sinkhole in the middle of  the roadway.

Sinkholes can form naturally through erosion. As water wears away underground rocks, empty spaces form. With nothing to support the surface above, it sinks. The collapse of underground structures like sewers can also create sinkholes, says Andrew Farrant, a geologist with the British Geological Survey. 

Officials say the sinkhole in Fukuoka may have been caused by nearby tunnel construction. Luckily, no one was injured. Engineers quickly filled in the hole with sand and cement. Certain types of rock are more prone to sinkholes, so investigating local geology before tunneling can help prevent them, says Farrant.