When flowing water hits against any kind of barrier, it twists away and spins around rapidly with great force. This creates a whirlpool. Whirlpools can occur in a small area where a piece of land juts out into a river, causing the water to swirl around.
They can also occur in the middle of the ocean when one current meets an opposing current, as when an incoming tide hits the ebb current of the last tide. Strong winds can also whip up the water into whirlpools.
The Maelstrom, a whirlpool off the coast of Norway, is one of the most famous, although writers such as Jules Verne, in 20,000 Leagues Under the Sea, and Edgar Allan Poe, in A Descent into the Maelstrom, have exaggerated its power. The Maelstrom is formed by the current hitting rocks and opposing tides.
In the United States, the Whirlpool Rapids of the Niagara River, between New York State and Canada, have such a violent current that they have actually carved a round basin out of the surrounding rock.