A building housing vacationers at a Florida resort collapses early Monday after a sinkhole opened up.
Though exploding glass windows and cracking sounds spooked guests at the Summer Bay Resort in Clermont, Fla., early Monday as a widening sinkhole caused one building to partially collapse, everyone escaped in time, said authorities.
Sinkholes have been happening for thousands of years, say geologists. Unlike the Clermont sinkhole, usually they are slow-motion processes that can take years.
When sinkholes do develop quickly, they can be fatal. In March this year, Florida resident Jeff Bush went missing after an apparent sinkhole opened in his bedroom in the middle of the night, sucking him and his bed deep into the earth. (After several days, search efforts for Bush were called off and he was presumed dead.)
Sinkholes like the two seen this year in Florida form when slightly acidic groundwater dissolves limestone or similar rock that lies beneath the soil creating a large void or cavities. When the overlying ceiling can no longer support the weight of the soil and whatever is on top of it, the earth collapses into the cavity.
If a house or road sits on top of the sinkhole, it too falls into the earth. At the Summer Bay Resort on Sunday night, the sinkhole appeared beneath an elevator shaft connected by breezeways to two apartment blocks.
Record July rainfall could be one possible trigger for the Summer Bay Resort sinkhole, said Grenville Draper, a geologist at the Earth & Environment department at Florida International University.
"I'm thinking the saturation of the ground makes the sand layer heavier, and this probably triggered the collapse of the sand into the cave," Draper told NBC News.
Sinkholes like these usually take a few hours to form, and he expects this one to get a bit larger before it stops.
The geology under central Florida makes that section of the state slightly more vulnerable to the sinkholes, Draper said. Though the two recent events have brought them into the spotlight, sinkholes are a natural phenomenon and have been collapsing for "thousands of years."
Sinkholes can reach more than 100 feet deep into the earth and spread across several hundred feet. Others are tiny — a few feet across and maybe a foot deep. Some hold water and form ponds.
"A very small percentage of sinkholes that form actually have some adverse effect on human life and infrastructure," Jonathan Arthur, director of the Florida Geological Survey, told NBC News. "However, it is those that make the news, whether it is under a roadway or a home."
Just because one sinkhole opens, does not necessarily mean another nearby is imminent. They are usually isolated events, the Florida Geological Survey notes. However, certain events such as a hurricane following a period of drought can trigger a series of sinkholes to occur within minutes to hours of each other, Arthur noted.
This happened in the wake of Tropical Storm Debby in June 2012. "It came across Florida after a period of drought where water levels in the ground were lower and then we had the massive influx of rain, over 20 inches in some areas, and that change in the climate and the groundwater levels triggered hundreds of sinkholes across the state over a very, very short period of time," Arthur said.
Human activity can also cause sinkholes to develop. Excessive pumping of groundwater, for example, can cause the soil to settle. Others form under the weight of runoff-storage ponds, which cause the underground support material to collapse.
In addition to Florida, other U.S. hotspots for sinkholes include Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania, according to the U.S. Geological Survey.
Though both the recent high-profile sinkholes have appeared at night, Draper said that this naturally occurring phenomenon can appear at any time.
John Roach is a contributing writer for NBC News. To learn more about him, check out his website. Nidhi Subbaraman writes about science and technology. Follow her on Facebook, Twitter or Google+.
First published August 12 2013, 8:40 AM