The Restless Sea by Robert Kunzig, W.W. Norton & Company. $24.95, 336 pages.

Kunzig writes for "Discover" magazine, and many of the ten chapters in this book first appeared as articles there. The result is a series of well-written, stand-alone pieces about oceanography -- covering everything from geology to jellyfish -- that are strung together without the overall chronological organization usually found in books about science. That cavil aside, the book is easy to read, and it will bring you up to date on the world beneath the waves.

Except for coastal shoals important to sailors, scientists had only a vague idea of sea-floor topography at the beginning of the twentieth century. Early oceanographers had to rely on individual soundings, each of which gave the depth of the ocean at only one point. In 1904, only 18,400 soundings had been taken in 142 million square miles of ocean. It was like trying to understand the geography of North Carolina by taking seven altitude measurements. Oceanographers of the day had no idea what the ocean floor looked like.

By the middle of this century, a few trenches and the mid-Atlantic ridge, an underwater mountain range that runs from South America to Iceland, had been crudely mapped. But only after multibeam sonar was invented in the 1960s, did oceanographers begin to make solid progress. Today, we have a comprehensive map of the sea floor. And what wonders it reveals.

Beneath the waves are globe-circling volcanic ridges far more massive than anything on land and 7-mile-deep trenches that could hide Mount Everest. Smaller seamounts -- but still tall enough to ski down if they were on land -- are sprinkled across what was once considered to be a featureless abyssal plain. Furthermore, the deep-sea floor, which had long been considered almost devoid of life, was found to harbor a surprising array of exotic creatures.

In 1977, scientists from Woods Hole Oceanographic Institution in Massachusetts took the submersible Alvin into the deep waters off the Galapagos Islands, searching for the epicenter of an underwater earthquake. What they found was a vent, a crack in the sea floor spewing hot water. And what they saw through the portholes amazed them. Clustered around the vent in the glare of Alvin's spotlight were "giant tube worms that grow in thick clumps with blood-red heads like flower buds . . . six-inch mussels and blind, porcelain-white crabs." They watched foot-long clams, "all thrown together in dense piles whose lushness rivaled that of any ecosystem ever seen before."

Biologists wondered what food source supported these organisms. The vents lie under a mile or more of icy water. Because sunlight doesn't penetrate those depths, photosynthesis cannot occur, so deep-sea organisms must rely on detritus, table scraps from the surface. Consequently, life on the sea floor is usually scarce and slow growing. What did the fecund life found around hot-water vents eat?
Some scientists posited that the vents created currents that concentrated food near them. But the real answer, far more astonishing, began to emerge when biologists collected some of the deep-sea organisms. When they opened the container, the stench of rotten eggs -- the odor of hydrogen sulfide -- permeated the laboratory. And in that sulfurous water were swarms of bacteria, organisms that had never seen sunlight.
A few bacteria on land live off hydrogen sulfide, but no one believed they could support an ecosystem. All ecosystems supposedly depended on photosynthesis. The vent communities, which are entirely independent of the sun, disproved this widely held theory. Today, some biologists believe that life on earth may have originated with the ubiquitous sulfur-eating bacteria in the vent-warmed waters at the bottom of the sea.

Kunzig does not confine himself to exotic organisms. The best chapter in the book deals with that most ordinary of fish -- the cod. He traces the history of cod fishing from its earliest days in the fifteenth century, when John Cabot reported that the Grand Banks, a shallow area off the Newfoundland coast, "was covered with fishes." So productive were the Banks that the fishery was deemed "inexhaustible" by Thomas Huxley, an eminent nineteenth-century biologist. Unfortunately, the eminent biologist was soon proved wrong.

Aided by the development of trawlers and factory ships, fishermen were soon taking incredible numbers of fish. In 1968, 810,000 tons of cod were netted on the Grand Banks, ten times the annual total caught by hook and line at the beginning of the twentieth century. Political pressures and overly optimistic estimates of the cod population encouraged Canadian fisheries officials to set harvest limits too high. Predictably, the take began to drop. By 1991, the cod catch was less than 20 percent of that of the halcyon days of the 1960s, and in 1992 the Grand Banks were closed to cod fishing, throwing 30,000 Newfoundlanders out of work.

This ecological and economic disaster was repeated in the United States when, at the insistence of commercial cod fishermen, Georges Bank off the Massachusetts coast was also overfished. One scientist who tried to stop the slaughter said that he defined his job at the council that oversaw the fishery as "A Thousand Ways of Saying, You're Killing too Many Fish." Unfortunately, nobody listened. Today, cod populations at the Grand Banks and Georges Bank show little signs of recovering, and both remain closed to fishing.

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Sidebar: Several books about the sea have hit the bookstores this year. "The Octopus's Garden," by Cindy Lee Van Dover, gives a more complete account of the fantastic life clustered around deep-sea vents. "Sea of Dreamers" by Phil Trupp introduces the reader to some of the deep-sea pioneers who explored the ocean floor. "Cod" by Michael Kurlansky, which was published in 1997, gives an in-depth look at the history of North Atlantic cod fishing and the demise of that fishery.