An undertow is a type of ocean current which is caused by waves breaking on the shore. Most undertows are quite mild and not dangerous, as long as swimmers keep their heads, although some have been known to be powerful enough to sweep swimmers out to sea. Incidentally, an undertow is not the same thing as a rip current, although the two currents are both caused by breaking waves on the shore.

To understand how an undertow works, think about what happens when waves break on shore. The water obviously has to go somewhere, and this is what causes an undertow: as waves break, water from previous waves runs underneath them, creating a gentle current which runs back out to sea. When there is heavy wave action, the undertow may not be able to get out, and as a result the water builds up and looks for a weak point in the breaking waves. When the water finds a weak point, it pushes out to sea, creating a rip current.

As you can see, the difference between rip currents and undertows is significant. Most undertows are not very strong, and the risk of an undertow is most severe for inexperienced swimmers who are standing or swimming near breaking waves. An undertow can pull someone underwater for a few seconds, but if the swimmer remains calm and swims towards the surface, he or she should be ok. The undertow is not usually strong enough to prevent the swimmer from returning to shore, unlike a rip current, which will carry the swimmer out to sea.

When swimmers encounter strong undertows, the tempting thing to do is to push towards the shore in the hopes of breaking through the undertow. This is actually a terrible idea, as swimmers can tire themselves out before they reach the shoreline. The best thing to do is to swim parallel to the shore, testing for a weak point in the undertow which will allow the swimmer to get back to shore, and the same technique works for rip currents. If a swimmer tires, he or she should tread water and float in the hopes that a rescuer will arrive soon.

If you are visiting an area with unfamiliar waters, it is a good idea to ask about prevailing currents. Locals can warn you about areas with especially strong undertows, or areas where rip currents often arise. As a general rule, the steeper the slope that the waves approach on, the stronger the resulting undertow; long shallow approaches create much less wave velocity, generating a much more gentle undertow.