IRA FLATOW, HOST:
This is SCIENCE FRIDAY. I'm Ira Flatow. By now I'm sure you've heard about the real-life nightmare of a Florida man named Jeff Bush. As he lay sleeping last week, a gaping hole opened beneath his home, swallowing him alive. His body was never found. The search has now been called off, and the sinkhole that devoured him is now his grave.
It's not the first dramatic sinkhole to hit Florida. In 1981, a sinkhole in Winter Park gobbled up an entire swimming pool, a camper and five Porsches at an auto repair shop. It was the size of a city block.
Why couldn't these events have been predicted, the sinkholes found before they get hungry? Why don't we find them? Do we have the technology to do that? Can you engineer a house so it won't fall into a sinkhole? In the last week, we've seen several appear. So is there a sinkhole season? Our number is 1-800-989-8255. We'll be talking about sinkholes.
First up, Randall Orndorff, a geologist at the U.S. Geological Survey in Restin, Virginia, also director of the Eastern Geology and Paleoclimate Science Center there. Welcome to SCIENCE FRIDAY, Dr. Orndorff.
RANDALL ORNDORFF: Thank you.
FLATOW: Where do these sinkholes come from?
ORNDORFF: Well, sinkholes occur in - when you have some kind of voids underneath the ground in the subsurface. Generally as a natural environment, they'll occur in areas where - what we call karst, or under lands that have soluble rock. So when you have a void underneath the ground, you know, when the rock dissolves, most of the groundwater is a little bit acidic, you're going to be able to pipe the soil from above at the surface into these voids.
FLATOW: So you have a kind of rock that is dissolvable by rainwater, or a little acidic rainwater?
ORNDORFF: Yes, you know, rainwater naturally is a little bit acidic as it is. And then as it works its way through the soils and into the subsurface, into the rocks, it picks up other organic materials and makes it a little bit more acidic. So rock types like limestone, gypsum and salt are three that come to mind that tend to be quite soluble.
FLATOW: And why are we hearing about them in Florida? Do they not strike any other places in the country?
ORNDORFF: Well, about 20 percent of the U.S. is underlain by rocks that have the ability to dissolve. But Florida is almost entirely underlain by limestone. So it tends to be a little bit more susceptible. Add along with that rainfall, fairly high rainfall, yearly rainfall, and you have the makings for sinkholes.
FLATOW: Was this sinkhole so deep that we really just had no chance of finding the man and getting him out? Do they get deep like that?
ORNDORFF: Absolutely. Depending on how deep the bedrock is, the thickness of the soil. In this case, I believe - I think I heard it was somewhere around 60 feet. That's not unusual. They can be a lot shallower. And depending on what the depth is to the ground - what we call the water table, where that water is underground will help determine how deep that might be.
FLATOW: So they can get a lot deeper, then?
ORNDORFF: Absolutely. There are places in the world where you can see them 100 feet and more.
FLATOW: A hundred feet. Is there - we're seeing like a cluster of sinkholes happening. Is there a sinkhole season?
ORNDORFF: Not necessarily. Sinkholes can happen for many different reasons at different times. However, we do tend to see sinkholes occur more often during high precipitation events, so during a wet season, for instance. The - if you have a void underground and in the soil it tends to work its way toward the surface. And so then you have - and we don't necessarily see any kind of subsidence on the surface if it's a nice, tight, clay soil.
But when you add a lot of water to it, you're adding - you know, water weighs a lot. So you're adding that weight to that soil bridge, basically, over that void. And once you get to a certain amount of weight, it'll collapse. The other situation is during droughts. Water tables will drop, so that'll give you a little bit more instability for what's on the surface, but also clay holds water very well.
And during a drought, you might dry up the water that's in that clay. And clay, you know, that water is basically like a glue. It's adhesive. And if you lose that adhesion, you also can get sinkholes.
FLATOW: Is there a way to detect if you have a sinkhole under your property someplace?
ORNDORFF: Not very easily. You know, detection, we'd love to be able to see in the subsurface, kind of like we try to see inside our bodies when you go to the hospital and have an MRI or a CAT scan, where you can actually see what's in play, which is uninvasive. We have what we call geophysical techniques like ground-penetrating radar and resistivity type of things to try to do that.
But we - it hasn't been completely successful, and then also you can only see so much, and it's probably as good as the time you did the test. The best way to detect is if you see changes: if you have a house or buildings, looking for cracks, see some kind of subsidence, if you see your door frames changing, things like that, and outside on the property looking for cracks in soil and things like that.
FLATOW: Well, thank you for all that information. And we'll be paying attention. Thank you, Dr. Orndorff.
ORNDORFF: You're welcome.
FLATOW: Randall Orndorff, a geologist at the U.S. Geological Survey in Restin, Virginia. Transcript provided by NPR, Copyright NPR.