All waves, no matter how huge, start as rough spots—cats' paws—on the surface of the water. The cats' paws are filled with diamond-shaped ripples, called capillary waves, that are weaker than the surface tension of water and die out as soon as the wind stops. They give the wind some purchase on an otherwise glassy sea, and at winds over six knots, actual waves start to build. The harder the wind blows, the bigger the waves get and the more wind they are able to "catch." It's a feedback loop that has wave height rising exponentially with wind speed.
Such waves are augmented by the wind but not dependent on it; were the wind to stop, the waves would continue to propagate by endlessly falling into the trough that precedes them. Such waves are called gravity waves, or swells; in cross-section they are symmetrical sine curves that undulate along the surface with almost no energy loss. A cork floating on the surface moves up and down but not laterally when a swell passes beneath it. The higher the swells, the farther apart the crests and the faster they move. Antarctic storms have generated swells that are half a mile or more between crests and travel thirty or forty miles an hour; they hit the Hawaiian islands as breakers forty feet high.
Unfortunately for mariners, the total amount of wave energy in a storm doesn't rise linearly with wind speed, but to its fourth power. The seas generated by a forty-knot wind aren't twice as violent as those from a twenty-knot wind, they're seventeen times as violent. A ship's crew watching the anemometer climb even ten knots could well be watching their death sentence. Moreover, high winds tend to shorten the distance between wave crests and steepen their faces. The waves are no longer symmetrical sine curves, they're sharp peaks that rise farther above sea level than the troughs fall below it. If the height of the wave is more than one-seventh the distance between the crests—the "wavelength"—the waves become too steep to support themselves and start to break. In shallow water, waves break because the underwater turbulence drags on the bottom and slows the waves down, shortening the wavelength and changing the ratio of height to length. In open ocean the opposite happens: wind builds the waves up so fast that the distance between crests can't keep up, and they collapse under their own mass. Now, instead of propagating with near-zero energy loss, the breaking wave is suddenly transporting a huge amount of water. It's cashing in its chips, as it were, and converting all its potential and kinetic energy into water displacement.
A general rule of fluid dynamics holds that an object in the water tends to do whatever the water it replaces would have done. In the case of a boat in a breaking wave, the boat will effectively become part of the curl. It will either be flipped end over end or shoved backward and broken on. Instantaneous pressures of up to six tons per square foot have been measured in breaking waves. Breaking waves have lifted a 2,700-ton breakwater, en masse, and deposited it inside the harbor at Wick, Scotland. They have blasted open a steel door 195 feet above sea level at Unst Light in the Shetland Islands. They have heaved a half-ton boulder ninety-one feet into the air at Tillamook Rock, Oregon.
There is some evidence that average wave heights are slowly rising, and that freak waves of eighty or ninety feet are becoming more common. Wave heights off the coast of England have risen an average of 25 percent over the past couple of decades, which converts to a twenty-foot increase in the highest waves over the next half-century. One cause may be the tightening of environmental laws, which has reduced the amount of oil flushed into the oceans by oil tankers. Oil spreads across water in a film several molecules thick and inhibits the generation of capillary waves, which in turn prevent the wind from getting a "grip" on the sea. Plankton releases a chemical that has the same effect, and plankton levels in the North Atlantic have dropped dramatically. Another explanation is that the recent warming trend—some call it the greenhouse effect—has made storms more frequent and severe. Waves have destroyed docks and buildings in Newfoundland, for example, that haven't been damaged for decades.