Charybdis was a maiden-turned-giant whirlpool in "The Odyssey" who sucked ships down to their deaths. Sounds tragically unscientific, but this ancient Greek epic got one thing right - how vortex physics work.
Originally Charybdis was a beautiful naiad, the daughter of Poseidon, who was punished for angering the gods. It's tough to feel sympathy for her, though, since unlike people who angered the gods by being born, being pretty, or weaving, she went around flooding villages to claim Zeus's territories. That's just dumb.
As punishment she was turned into a hideous, perpetually thirsty monster. She lay on the bottom of the ocean and every day she would try to drink the whole thing. Sucking down that much water caused a whirlpool that would drag down ships.
Physics-wise, the ancient Greeks got it right. The first step to creating a decent whirlpool is taking the bottom out of the container. Or, at least, a part of the bottom.
Two things will generally happen. The water will rush downwards, and the air (or other material) below will rush up. This could happen just by the air bubbling up, accompanied by rude noises, and the water glugging down in fits and spurts. However, under the right conditions, whirlpools form.
The water, when tumbling down into the hole, doesn't necessarily tumble straight in. The entire pool of water is affected, and some of it will not go directly down the hole. Instead it rushes toward the hole and gets muscled slightly off to one side, either by other water or by air escaping upwards, its momentum making it overshoot the mark. It's still being drawn back to the hole though, so it gets drawn back and sideways. If it overshoots again, it's drawn back and sideways again. Eventually it starts circling.
Once water starts circling, it pulls and pushes other water along with it. Soon even water that isn't subject to an immediate downward pull can get drawn into whirlpool. One of the most spectacular examples is Lake Peigneur.
When an oil-drilling team on the lake hit a salt mine beneath it, the ensuing whirlpool sucked up not only the water, but a good bit of the land around it as well. Puts Charybdis to shame.
[Via How Stuff Works.]
Top image by William Pye.