In physical oceanography, undertow is a subsurface flow of water returning seaward from shore as result of wave action. This type of shore current can play a role in material deposition such as creating sand bars.
Undertows are below-surface rushes of water returning to sea after the water comes ashore as breaking waves. If there is an area under the waves where water can flow back out to sea more easily (such as a break in a sand bar) then a narrow rip current can form. (A rip current is much more powerful and thus more hazardous to inexperienced people than ordinary undertow.) If however there is no weak point in the surf line, then the water that has run up onto the beach simply flows back out to sea under the waves, forming a simple undertow.
There is a popular misconception that the undertow is responsible for many drownings by somehow pulling people vertically down, so that they are then held under the water surface. While people can drown in any body of water, even a puddle, undertow itself is relatively harmless; the great majority of drownings near the beach happen because people get caught in a rip current which draws them out to sea, and they are either poor swimmers or do not understand how to get free from a rip current.