There was a bit of unpresidential excitement at Mar-a-Lago when the earth opened up in front of the estate. A newly-installed water main is the apparent cause of the shenanigans, which left a four-by-four foot hole in Southern Boulevard. This video gives you a good look at it, and a nice view of its context:

Everyone's calling it a sinkhole, but it isn't actually one, geologically-speaking. So what is it? Let's explore!

Did you notice how narrow the barrier island Mar-a-Lago's built on is? This is our first clue: sinkholes require some rock-like substrate that water can dissolve, like salt or limestone or other kinds of evaporite or carbonate rock. And one thing a barrier island isn't is rock: it's mostly sand. Not even sandstone. Just sand.

Barrier islands aren't exactly karst paradises. And that fact is reflected in Palm Beach County's sinkhole activity: there ain't much. If you look at the sinkhole hazard map for Florida, you'll see that it's in the green zone: a swath of Florida that's buried 20-200 feet deep in sand. It's definitely not karst, but it's prone to its own problems, as we'll soon see.

Our next clue comes from the public notice put out by the city- of Palm Beach. They note that the "sinkhole" seems to be near a water main that was recently installed. Ah-ha. You know what water mains often do? They leak. Sometimes they burst. And when they do, they tend to wash away the sand and soil beneath the surface, leaving a void which streets, vehicles, and sometimes structures can fall right into.

Image is taken from an upper floor in a building, looking over a concrete street at night. A large, square hole has opened, with slabs of the treet and sidewalk buckling and sinking along its edges. One of the fancy lampposts has almost fallen in. Water fills the hole. Officials in yellow jackets and white hardhats stand near the edge, assessing it while a person in a dark suit gestures at the hole.
A water main break caused an appreciable chunk of Montrose Avenue in Chicago to fall into the cavity created by rampaging water. Credit: Erin Nekervis Flickr (CC BY-ND 2.0)
 

So what is this, if not a sinkhole? It's a feature called a collapse. But most folks call them sinkholes, and they're really similar to sinkholes in the kind of damage they cause, so there's no need to go around yelling "It's not a sinkhole!" every time one opens up.

Now, water main breaks can cause some serious problems you won't necessarily get with a run-of-the-mill sinkhole. So it pays to familiarize yourself with the signs of a water main leak or an impending break. If you notice any of those signs, call your city's hotline for utility emergencies and get to safety. And if you're caught in a fairly major water main break, remember something really extremely important: if there's gas leaking, you may not be able to smell it.

Gas leaking from a damaged gas main can migrate underground into the surrounding buildings, the sewer system, and the electric service manholes. Don't forget that the gas odorant mercaptan can be scrubbed out of the gas as it passes through soil, so the absence of a gas odor does not mean there is no migrating gas in a building.

Yikes. Looks like Mar-a-Lago got lucky. Their hole wasn't caused by a full water main break, but probably from some leaking or subsidence caused by the construction.

Collapses caused by water main breaks may also be concealed by the gargantuan volumes of water filling them, so don't assume the huge puddle in the road is shallow: all that water may be hiding a deep dark hole. Drive carefully!

The other bit of bad news: collapses can happen to property owners everywhere. They're caused by more than water mains: something as simple as a stump that was never removed might leave a void that bits of your property will suddenly collapse into. So know what to look for, how to fix it, and how to get help if the job is too big for just you.

So what if, in addition to being at risk of sudden collapses, you're also living in an area at risk of suffering real live sinkholes? In that case, the best armor is information! There are some excellent free resources online: this Lake County, Florida website is a fabulous introduction to sinkholes and how to live with them, and there's a much more detailed PDF from Pennsylvania's Department of Conservation and Natural Resources. One of the best resources I've ever found is the book Living With Karst:  A Fragile Foundation, which packs a remarkable amount of accessible information in a very slim volume. Everyone living in karst country should have a copy.

Mar-a-Largo's probably not at risk of falling into a sinkhole, unlike many other structures in Florida, but if that new water main ever breaks, we'll certainly be seeing it in the news again. And now you'll know about the geology beneath it and the physics behind it!