Few things can top a day at the beach on a warm day. The sunshine is out in full force and crashing waves beckon when it is time to cool off.

While there is much fun to be had at the beach, some dangers lurk as well. Though rip tides or jellyfish may come to mind, the beach itself, namely the sand, also poses a threat.

Young children and even adults bring shovels and buckets to make sand castles and even to dig holes in the sand to cover friends and family members. Such holes can quickly cave in and trap unsuspecting people inside. That’s just what happened to 12-year-old Reno Ciotola in 2017 in Ocean City, Maryland. Ciotola dug a tunnel in the sand when it suddenly collapsed, trapping him underneath. Lifeguards were able to dig him free eventually, but Ciotola was unconscious with no vital signs. Luckily he was revived. It’s not the first nor the last case of beach sand cave-ins.

According to Stephen Van Ryswick, chief of the Coastal and Environmental Geology Program, holes dug in beach sand react differently than holes dug in fields. The “angle of repose” is much lower in sand. This is the maximum angle an object can rest on at an incline without sliding down. Beach sand has an angle of repose of approximately 30 degrees, according to Van Ryswick. By comparison, rock can maintain a 90-degree angle of repose.

Holes are typically dug where sand is moist. The water in the sand initially holds the sand together more firmly, but as it evaporates and the sand dries and gets disturbed, collapses can occur. Bradley Maron, a Harvard University researcher, says he’s tracked 72 sand hole collapses over the past decade. Of them, 60 have been fatal.

Lifeguards are increasingly recognizing sand digging safety as a concern. They are being trained in rescues and warning beachgoers about the risk of sand collapse. Experts warn that a good rule of thumb for digging sand holes is not to make them any deeper than the knees of the smallest person in the hole.

Danger lurks at the beach, and it isn’t always in the water. Sometimes the sand can be hazardous as well.