(Or see maelstrom first if you don't understand)
Usually circular, it can also be dangerous. If you get too close, you may be grabbed and sucked into the center. Sticks and other rock hard debris may penetrate your hull. Malestroms may also involve large amounts of fast-moving liquid.
Guy: "Ohhh yeah!"
Girl: "So you had fun, then?"
Guy: "Bet your ass, I had fun - it was one fucking malestrom!"