Shark attacks and rip tides are understandable worries at the beach, but parents need to be aware of another, more likely danger to their children — collapsing sand holes.

In the last 20 years, at least 20 U.S. children or young adults have died when they became buried in sand after the rapid collapse of a sand hole.

Bradley Maron, M.D., a former lifeguard and now a physician at Harvard Medical School, has identified 16 sand-related deaths from 1990 to 2006 compared with 12 fatal shark attacks during that period, based on statistics from the University of Florida. Most of the victims who were smothered by sand were boys ranging in age from 3 to 21 years.

You or your child is of course unlikely to become a sand-hole fatality, but Maron and others advise parents to keep a watchful eye on young children, whether they are playing in the sand or in the ocean. Maron also advises that no one get into a hole deeper than their knees.