People riding a bus in Pittsburgh, Pennsylvania, recently hit an unusual snag during their commute. Their vehicle fell into a 23 meter (75 foot)-wide sinkhole when the street beneath the bus suddenly caved in.
Sinkholes can form naturally through erosion as water wears away rocks underground. Eventually, the ground can no longer support the surface above, and it sinks. Sinkholes can also form in cities if the earth beneath a structure is disturbed, says Daniel Bain, a geologist at the University of Pittsburgh.
A broken water main, for example, caused the sinkhole in Pittsburgh. The pipe released water, which washed away soil and other sediment below the street. “When something heavy like the bus drove over it, the roadway collapsed,” says Bain. Luckily, no one was seriously hurt.