A whirlpool is a swirling body of water usually produced by ocean tides. The vast majority of whirlpools are not very powerful. More powerful ones are more properly termed maelstroms. Vortex is the proper term for any whirlpool that has a downdraft. (Technically, these approximate to a 'free vortex', in which the tangential velocity (v) increases as the centre line is approached, so that the angular momentum (rv) is constant). 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, like Niagara Falls, these whirlpools can be quite strong. The most powerful whirlpools are created in narrow shallow straits with fast flowing water. A whirlpool in the southern hemisphere travels counterclockwise and a whirlpool in the northern goes clockwise

The five strongest whirlpools in the world are the Saltstraumen outside Bodø in Norway, which reaches speeds of 37 km/h; the Moskstraumen off the Lofoten islands in Norway (the original maelstrom), which reaches speeds of 27.8 km/h; the Old Sow in New Brunswick, Canada, which has been measured with a speed of up to 27.6 km/h; the Naruto whirlpools in Japan, which have a speed of 20 km/h; and the Corryvreckan in Scotland, which reaches speeds of 18 km/h.

Powerful whirlpools have killed unlucky seafarers, but their power tends to be exaggerated by laymen. There are virtually no stories of large ships ever being sucked into a whirlpool. Tales like those by Paul the Deacon, Jules Verne and Edgar Allan Poe are entirely fictional. The closest equivalent might have been the short-lived whirlpool that sucked in a portion of Lake Peigneur in New Iberia, Louisiana, USA after a drilling mishap in 1980. This was not a naturally-occurring whirlpool, but a man-made disaster caused by breaking through the roof of a salt mine. The lake then behaved like a gigantic bathtub being drained, until the mine filled and the water levels equalized. Although some boats and semi trailers were pulled into it in the classic whirlpool stereotype, no human lives were lost.

In popular imagination, but only rarely in reality, whirlpools can have the dangerous effect of destroying boats. In the 8th century, Paul the Deacon, who had lived among the Belgii, described tidal bores and the maelstrom for a Mediterranean audience unused to such violent tidal surges:

Not very far from this shore... toward the western side, on which the ocean main lies open without end, is that very deep whirlpool of waters which we call by its familiar name "the navel of the sea." This is said to suck in the waves and spew them forth again twice every day...