Why Do Sinkholes Keep Opening up in This Florida Neighborhood?

Sinkholes occur in Florida more frequently than any other U.S. state. But over the past week, at least a dozen of these holes in the ground have opened up in just a single neighborhood in the central city of Ocala, 80 miles northwest of Orlando, according to local media reports.

Residents from at least eight families in the Fore Ranch subdivision of the Wynchase Townhomes neighborhood were evacuated after a number of sinkholes formed around a retention pond—a pool designed to contain stormwater runoff from roofs and streets.

Resident Maren Pinder told ABC affiliate WFTV that the holes kept appearing and were growing larger, sparking worries that they may combine to form a mega-sinkhole.

"They just keep coming. Are we safe? We don't know. It's really scary. We just have a bag ready in case they say, 'Yeah, you need to evacuate.' We can just grab it and get out," Pinder said.

No one has been injured so far, and all the buildings in Fore Ranch are still standing, but the holes have swallowed several cars, according to the Miami Herald.

David Wilshaw, principal geologist with Florida-based company Britannia Solutions, which advises engineers and architects on ground risk management, told Newsweek that both the location of Fore Ranch and the construction of retention ponds in the area are important to understanding why the sinkholes formed.

"The area of Ocala where the Wynchase Townhomes neighborhood is located was dry, well-drained, open pasture land as recently as 2003," he said. "The shallow soils in this area tend to be sands, below which there is generally a layer of more impervious clay soil that lies above the limestone bedrock. And the limestone in Ocala can be very cavernous. As part of the development of the subdivision after 2003, stormwater management ponds were excavated.

"Unfortunately, digging these ponds decreases the thickness of soil between the ground surface and the limestone—shortening the 'fuse' for a sinkhole to form," Wilshaw added. "Furthermore, directing all stormwater into the ponds and creating these artificial lakes concentrates seepage of water into the soils. Such seepage is a destabilizing force in sinkhole formation."

But while recent development has played a significant role, the process that creates the initial caverns in the limestone has been taking place for thousands of years, Wilshaw added.

"Caverns form in limestone very slowly—the rock dissolves in acidic water at a rate of about 1 inch per 1,000 years. So the holes that exist in the limestone were there long before the subdivision was developed," he said.

Once instability occurs in the soils that sit above these caverns in the limestone, the conditions are created for sinkholes to form. This was demonstrated in 2012 in the aftermath of Tropical Storm Debby, when 14 sinkholes opened up in the retention pond immediately to the south of the pond with the current problem. The additional runoff from the storm was enough to cause the clay soils lining the pond to crack and fail at multiple points.

This kind of instability also likely led to the sinkholes that formed around the retention pond near Fore Ranch over the past week.

"The difference is that we have not had a storm," Wilshaw said. "However, we are in a dry spell. As water levels in the ground drop during the dry season, the soils lose the buoyant support provided by the groundwater—they 'feel' heavier. A bit like if you are sitting in the bathtub, then you drain out the water—it's harder to get out of the bath!"

A sinkhole on Scenic Highway after part of the highway collapsed following heavy rains and flash flooding in Pensacola, Florida, on April 30, 2014. Over the past week, at least a dozen sinkholes like these have formed in a Florida neighborhood. Marianna Massey/Getty Images

If the sinkholes are left untreated, there is a danger that the residents' worst fears could materialize and the holes will coalesce to form larger sinkholes, Wilshaw said.

During the recent events, the residents of Fore Ranch have complained that they have often been left in the dark by officials. "We have not received any answers," Shannon Cole, told local media outlet Ocala.com. "I'm going to have to find somewhere [to live] soon."

The Ocala Fire Rescue Department and the city's engineer, as well as the county's Emergency Management Department, have been investigating the sinkholes to try to prevent them from happening again.

However, these efforts are unlikely to ease the concern of those living in the area.