Summer is here, and if you’re able to make it to a beach, you’ve got a lot to look forward to: fresh air, crashing waves, and finding sand in your car for the next several days.

One popular activity for beachgoers is digging holes in the sand. Kids can even buy tiny little shovels and pails and pretend they’re utility workers excavating broken water lines. But burrowing into sand isn’t as harmless as you might think. At least one site, Bethany Beach in Delaware, has even banned people from digging.

The reason? Holes can collapse without warning and trap visitors. Wet sand that seems structurally sound when it’s moved can dry out, leading to crumbling. And because sand is heavier than most realize—it’s about 100 to 112 pounds per cubic foot—it can virtually seal someone in place and cause enough pressure on the chest to inhibit breathing.

Firm statistics can be hard to come by, but Bradley Maron, an assistant professor at Harvard Medical School, told The Washington Post in 2014 that he had found at least 77 cases of emergencies, both fatal and non-fatal, as a result of sand digging.

In 2018, a visitor to Crescent Beach in Florida was seriously injured after a sand hole collapsed around him. In 2012, a man on Rehoboth Beach in Delaware needed rescue after a connecting tunnel he dug fell apart.

Owing to umbrellas and other obstacles, lifeguards may not see someone is in danger until it’s too late. And if someone digs a deep enough hole and it collapses, rescuers might not be able to make them out—the sand will look uniform.

If you must dig, a good rule of thumb is to dig only as deep as the height of the knees of the smallest person in the group. And never allow kids to try and “tunnel” from one hole to another. The inevitable cave-in could be life-threatening.

Even if the hole isn't deep, it can still be a hazard. People can trip over it, and beach rescue vehicles might get stuck while racing to attend to an emergency.

If you spot a person in trouble in sand, it’s best to seek help immediately—frantic digging can actually make the sand shift and the problem worse.

It’s not just human visitors that can be at risk. A recent tweet from Maine Audubon reminded people to fill any holes dug on the beach before they pack up and go. The reason? Human-made sand trenches pose a big problem for juvenile shorebirds, who may find themselves in the sink without any way of getting out because they're too young to fly.

To drive the point home, the Audubon tweet included a picture (courtesy of Suzanne Nobile) of an adorable—and endangered—piping plover chick who didn’t have the strength to get out of a hole without assistance and had to be rescued. Worse, some beach-nesting birds, like the plover, can blend into the sand to the point where passers-by might not even notice them. Sea turtle nestlings can also become trapped.

So for both the safety and humans and their animal pals, opt against digging, or at least keep it to a minimum. Who wants to work that hard on a beach, anyway?