If the dirt around your home’s foundation seems to be separating from the house, or if you’re noticing a lot of cracks on your indoor walls around windows and doors, the problem could be the soil under your house.

More than half of homes around the country—and more in Arizona—are built on soil that’s considered “expansive”—that is, it swells when it gets wet and shrinks when it gets dry—and about half of them will suffer some damage because of that swelling and shrinking, the U.S. Department of Agriculture reports.

Geologists estimate that soil swelling and shrinking causes more damage to homes and other buildings than hurricanes, earthquakes and floods combined during an average year.

Much of the soil we build our homes on in the desert includes lots of clay, which is prone to swelling and shrinking. When it gets wet, it swells and “heaves,” when it dries out again, it shrinks and “settles.”

So does whatever is on top of it—including your home.

When your home heaves, that means the part of the foundation that sits on top of the swelling soil literally rises above the rest of the foundation. When it settles, part of the foundation drops below the rest of it. This causes those cracks that you see, but it also can cause structural damage to the home that you can’t see.

It’s better to take care of it before it becomes a huge problem. How? Consult with a geotechnical engineer, whom you can find through a company that resolves heaving and settling problems. Two I can personally recommend: Arizona Ram Jack in Phoenix and Marana; and Arizona Helical-Piers Corp. in Cave Creek.

If you’re shopping for a home, it’s a good practice to find out how “expansive” the soil is under the one you hope to buy. Ask the sellers to tell you. If they don’t know, consult with an engineer. It will be worth the money you pay for the consultation to know whether you’re investing your life savings in a property that could cause some costly trouble down the road.

Most people, in fact, don’t know what kind of soil is under the home because it’s possible live in a house for years and years without ever noticing any signs of a foundation problem. That’s because the damage can occur slowly.

When it does, though, it can be major.

The good news is that not all clay-based soil contains enough of the kind of clay that will cause it to heave. And not all clay with the potential to heave will do it—as long as it doesn’t get wet.

Also, builders can prepare the soil on the site of a new home to prevent the problem from occurring. If you’re getting ready to buy a brand-new home, ask your builder about the soil and his plans to prevent heaving and settling.

It is possible to build safely on expansive soil. It’s also possible for the homeowner to help prevent heaving and settling by keeping the property dry.