After some highly publicized recent sinkhole disasters, such as the resort that collapsed near Orlando, Fla., some Americans are left to wonder: is the country caving in on itself?
The answer, according to experts, is predictably complicated. Sinkholes form as the sediment overlaying soluble bedrock sinks into underground holes. Often seen as "natural" disasters, sometimes sinkholes are caused -- or at least aggravated -- by human activity above the ground.
In Montreal recently, a huge piece of construction equipment fell into a sinkhole that occurred in a major traffic intersection. In Louisiana, a sinkhole measuring 422 feet deep and 273 feet wide opened up and forced a number of residents to evacuate their homes. In Florida, Tiger Woods' $60 million mansion is reportedly in danger of sinking. The reasons for all of these crisis points have much to do with people.
"The human influence on these environments is undeniable," said Randall Orndorff, director of the Geology and Paleoclimate Science Center at the United States Geological Survey (USGS).
"In the natural environment we come in and start developing these areas, putting up parking lots, roadways, buildings, where you have roofs that collect water in gutters and run off in parking lots and roads, and then into ditches. All of the water that used to go into the soil is concentrated in these storm drains, basically creating big gushers. We're changing the hydrologic regimen."
The predisposition for sinkholes is simple, said Orndorff: Start with soluble bedrock, such as limestone, then overburden the surface above, allowing the soft sediment to collapse into it.
For example, the combination of a drought followed by a tropical storm is the ideal recipe for sinkholes. If the clay cap over the limestone severely dries out, and then is suddenly pummeled with rain, adding weight, and if there is already a cavity formed underground, the structure above it is likely to cave in.
The hard part is knowing in advance what areas are most vulnerable to sinkholes.
"You have to consider the influence of urbanization," Orndorff said. "The paving over of areas and competing land uses, sitting right next to an agricultural area that has been pumping water out of the ground for years, sets up a situation for sinkholes."
In Florida, for example, where there have been many reported sinkholes, the population has grown tremendously over decades, and with all those people creating buildings, roadways, and suburban sprawl -- it drives the water system too hard, and the water tables drop. When there's a drought they pump a lot of water. When there's a freeze they cover the plants with water to protect them.
Determining if a land area is prone to sinkholes before building on it can be laborious and cost prohibitive, so often it is not checked out.
"The problem occurs if you build over an area that is prone to sinkholes," said Terry R. West, Ph.D., P.E. and professor of Earth sciences at Purdue University. "The builder would have to drill holes in the ground and look for cavities or make a geophysical survey and locate the cavities underground with seismic techniques. That would be out of the ordinary and expensive, and is not usually done unless you suspect already that the property is prone to sinkholes."
Although sinkholes seem to be reported most frequently from Florida, they can and do happen almost anywhere.
"We had one in Indianapolis due to a leaking water pipe," said West, author of "Geology Applied to Engineering." "Also, in Indiana there are mines that run very deep into the ground from 100 years ago, and that poses a threat."
Decaying infrastructure nationwide is indeed a contributing factor, said Orndorff.
"Just a few weeks ago a woman drove her car right into a sinkhole in Toledo, Ohio," Orndorff said. "It is a real concern for our country that the water mains and storm drains in big cities were built in the 1800s. Some of what we're seeing is not necessarily just geologic -– the sinkholes are related to the aging infrastructure. The size of the sinkhole has everything to do with how big that void is beneath the surface of the city.
The good news in the midst of all of this devastation is that sinkholes are not necessarily instantaneous.
"Sometimes there are warning signs weeks or months in advance," Orndorff said. "If you see signs of settling in your house, windows that are skewed, doors that won't close or cracks in the walls, or even small depressions in the land around your house, you should call your local jurisdiction. Usually they will send in a geo-technical engineer."