If you've ever swum in the ocean you know that it is not a neutral environment. It looks manageable from the shore. You walk in, it's fine, you're swimming, you feel capable. And then at some point you look up and the shore is further away than it should be. You've been moving. You just haven't been moving in the direction you thought.

That's an undertow.

An undertow is not a wave. It's not dramatic. It doesn't announce itself. It's a current running beneath the surface, moving in a direction opposite to where you're trying to go, and it is most dangerous precisely because you can't see it from where you're standing. By the time most swimmers realize they're in one, they've already been carried.