Whirlpools

A whirlpool is a small "underwater" tornado. It is created by two currents or tides coming from different directions and colliding. The vast majority of whirlpools are not very powerful. More powerful ones are more properly termed malestroms. Vortex is the proper term for any whirlpool that has a downdraft. Very small whirlpools can easily be seen when a bath or a sink is draining, but these are produced in a very different manner from those in nature. Smaller whirlpools also appear at the base of many waterfalls. In the case of powerful waterfalls, these whirlpools can be quite strong. The most powerful whirlpools are created in narrow shallow straits with fast flowing water.

The five strongest whirlpools in the world are the Saltstraumen in Norway, which reaches speeds of 37 km/h (23 mph); the Moskstraumen off the Lofoten islands in Norway (the original maelstrom), which reaches speeds of 27.8 km/h (17.3 mph); the Old Sow in Eastport, Maine, United States, which has been measured with a speed of up to 27.6 km/h (17.1 mph); the Naruto whirlpools in Japan, which have a speed of 20 km/h (12 mph); and the Corryvreckan in Scotland, which reaches speeds of 18 km/h (11 mph).

Source: Wikipedia ^^