A horse died in Chandler Heights a few years ago when a tiny crack in the earth opened up during a monsoon thunderstorm and swallowed its corral. A consulting geologist I know says nearby homeowners are lucky the crack—called an earth fissure—didn’t open up under their homes and swallow them, too.

Hundreds of earth fissures—cracks at or near the soil surface caused by decades of removing more groundwater than the rain and snow can replenish—are scattered around Arizona’s valleys, and there’s no telling when they will transform from harmless cracks to huge gullies. Stay away from them, yet more homes and subdivisions are springing up too close for comfort.

The state Legislature has taken steps to protect would-be homeowners from unknowingly building or buying a home on a fissure: Sellers must disclose the presence of a fissure on the property, and the Arizona Geological Survey is set to release the most comprehensive map ever of the state’s known fissures.

Still, that won’t wipe out the double-whammy of a growing population with a growing need for water, because much of that water has to come out of the ground.

Here is the problem: The ground is sinking because we’ve pumped so much water for so long out of the underground aquifers. In fact, the state is pumping water out 100 times faster than nature can replenish it.

The result: Water tables have dropped by up to 350 feet, leaving the sand, gravel, silt or clay that makes up the soil too dry and compact to hold itself together. So it cracks and shrinks, and eventually, like a house of cards, it collapses. A torrential downpour—the kind we get most summers—hurries it along.

This wasn’t much of a problem years ago when the valleys were home mostly to large farms and just a few, scattered houses. Today, however, homes are all over the place—and some are dangerously close to fissures.

Some simple advice:

1. Find out if the property you own or the land you’re thinking about buying contains a fissure. Go to the Web site of the Arizona Geological Survey’s Earth Fissure Center (http://www.azgs.az.gov/EFC.html). You’ll find maps there that show where some of the state’s fissures are located.The Arizona Geographic Information Council (http://agic.az.gov) will post an updated map that shows every known fissure in Maricopa, Pinal, Pima and Cochise counties.
2. If you want to buy a home, study the seller’s real estate disclosure form before signing a contract. The owner has to reveal whether there are fissures on the property. If the owner doesn’t know, have a soils expert inspect the property before you buy.
3. Avoid building or buying a home on a fissure, even if it looks like only the slightest crack in the earth. There’s no way to tell when or even if it eventually will open up and swallow whatever is sitting on it.
4. Build or buy your home as far away from a known fissure as possible. Even if the obvious signs of a fissure are a mile away, that crack could extend underground for a long way. If a monsoon downpour caused it to grow from a crack to a gully, it could open up on the property around it.
5.  If there is a fissure on your property, keep it dry. Water is the catalyst that turns tiny cracks into huge gullies. Don’t irrigate your lawn. Direct the runoff from downspouts in another direction.