Where they occur: Off the coast of Japan, Norway, the USA and Scotland

Max size: 75m (250ft) in diameter – Old Sow, USA

Max speed: 27.8kmph (17.3mph) - Moskstraumen off the coast of Norway

Casualties: Tidal whirlpools do not have enough power to capsize boats, although the vortex could easily drown a swimmer or diver

As ocean tides rise and fall, they sometimes produce rotating currents known as whirlpools. Some whirlpools have a downward pull, known as a vortex.

Japan’s Naruto whirlpool is one of the world’s most famous examples. Found in the strait which connects the Seto Inland Sea to the Pacific Ocean, the Naruto whirlpool has the third fastest current in the world, at 20kmph (12.4mph), and a vortex of up to 20m (65ft) in diameter. Water rushes through the strait four times a day, creating a huge swirling whirlpool that attracts many tourists and observers.

The whirlpool with the world’s fastest current is the Moskstraumen, off the coast of Norway - its speed is almost matched by the wonderfully named Old Sow, which is between New Brunswick and Maine in the USA.