A whirlpool (or maelstrom) is a body of rotating water produced by opposing currents or a current running into an obstacle.[citation needed][clarification needed] Small whirlpools form when a bath or a sink is draining. More powerful ones in seas or oceans may be termed maelstroms. Vortex is the proper term for a whirlpool that has a downdraft.[citation needed] In narrow ocean straits with fast flowing water, whirlpools are often caused by tides. Many stories tell of ships being sucked into a maelstrom, although only smaller craft are actually in danger.[1] Smaller whirlpools appear at river rapids[2] and can be observed downstream of manmade structures such as weirs and dams. Large cataracts, such as Niagara Falls, produce strong whirlpools.