A burst sewer pipe or storm drain probably hollowed out the underground cavity that allowed the chasm to form, according to Sam Bonis, a geologist at Dartmouth College in New Hampshire, who is currently living in Guatemala City (map).

The Guatemala City sinkhole, estimated to be 60 feet (18 meters) wide and 300 feet (100 meters) deep, appears to have been triggered by the deluge from tropical storm Agatha.

But the cavity formed in the first place because the city—and its underground infrastructure—were built in a region where the first few hundred meters of ground are mostly made up of a material called pumice fill, deposited during past volcanic eruptions.

"Lots of times, volcanic pumice originates as a flow