Since tsuwm didn't give me a wwftd I could post about, I went to his list of them and picked out "aegir".
He defines it as a tidal bore. Not the all-wet kind of guy who drips on people at a party. My dictionary says it was originally a god of the sea.
I wept a tidal wave of tears when I was in the vicinity of the one that is part of the Bay of Fundy, but not at the right time to see it in action. I have seen pictures of a tugboat tied up to a pier at high tide, and resting on the bottom forty feet below at low tide. I have read that there is another in Korea.
Maybe Bean could give us some information about the natural elements necessary to create one.

Here is a good URL about them:http://www.newi.ac.uk/greens/bores.htm