The deep ocean is a world that seems as alien to human beings as a planet in a science-fiction novel. It is a realm of absolute darkness, where sunlight never penetrates and the frigidly cold water exerts pressures measured in tons per square inch. The ocean floor is a landscape of rolling plains, deep canyons, and dramatic mountain chains, all formed by vigorous geologic activity. Here, molten rock from deep within the Earth forms new areas of sea floor and hot springs called hydrothermal vents belch clouds of superhot water and minerals from deep within the crust into the surrounding water. The vents and the areas around them are home to a wealth of strange creatures—microbes that feed on sulfur compounds erupting from the springs; six-foot, red-tipped worms fed by colonies of bacteria living within their guts; and bizarre animals that resemble limp dandelions.

In August 1996, scientists reported that the vents harbor a form of life that is truly alien to our world of air and sunlight. Researchers at The Institute for Genomic Research (TIGR) in Rockville, Maryland, announced that they had mapped the entire genome (genetic code) of a single-celled organism called Methanococcus jannaschii. Taken from the sides of a vent 2 miles (3 kilometers) beneath the Pacific Ocean in 1982, the organism resembled true bacteria in that it was composed of a single cell that had no nucleus. Genetically, however, it was in many ways closer to plants and animals--organisms whose cells have nuclei. Furthermore, it lived under conditions that no bacterium could survive. The TIGR scientists concluded that M. jannaschii belongs to a third kingdom of organisms—literally, a new kind of life form—the Archaea. The significance of this discovery was not completely understood in 1997, but it was already causing a revolution in biological thinking. As one researcher remarked: "It shows how little we know about life on this planet."

It is remarkable, but not surprising, that we could have shared the planet with a third branch of life for so long while knowing nothing of its existence. Even as scientists study the features of Mars and Venus, the deepest recesses of our own ocean remain largely a mystery. The ocean covers more than 70 percent of the Earth's surface—more than twice as much as all the land masses combined—and yet human beings have explored only a tiny fraction of it. But that is changing. The pace of undersea exploration is increasing, and the benefits of studying the ocean are moving beyond the realm of theory. By the mid-1990's, many geological features of the ocean floor had been mapped and many of the unusual creatures that inhabit its depths classified. Meanwhile, oceanographers were developing new tools and methods for exploring the deep, including a variety of advanced submersible vehicles. We are embarked on a real-life voyage to the bottom of the sea that promises to change the way we view the ocean.