Some of the dead matter escapes the degrading microbes and sinks into the deeper, darker layers of the ocean. On the way down, it nourishes another population of animals—some fish, but also a huge array of gooey gelatinous things, known to their few scientific friends as jellies. Jellyfish proper, the medusae, are just one kind. They are familiar because they often venture into the shallows where humans paddle about. There are also the ctenophores, or comb jellies, with their eight rows of tiny rippling paddles; cylindrical salps that swim by jet propulsion; and floating snails that catch their food by casting large nets of mucus.
Many of these animals are able to light up like fireflies—whether to scare off predators or attract a mate is not entirely clear. Descending in a submersible from the sunlit surface waters into the deep and utter dark of the abyss, one sees these bioluminescent flickers, like flashbulbs in a darkened concert hall.
Then there is the bottom. The seafloor is not a single place; its topography is every bit as varied as that of dry land. A rugged chain of volcanic mountains, the midocean ridge, runs down the center of the Atlantic, around Africa into the Indian Ocean, between Australia and Antarctica, and across the South Pacific, then up the East Pacific to California, where it becomes the San Andreas Fault. The ridge rises as high as 15,000 feet above the surrounding abyssal plains. Here and there, those hilly plains are interrupted by underwater mountains called seamounts. In certain places along the rim of the oceans, especially the Pacific Rim, the seafloor descends abruptly into deep trenches. The most extreme, the Mariana Trench near the Philippines, plunges nearly seven miles, far deeper than Everest is tall. In 1960 Swiss explorer Jacques Piccard and American Navy lieutenant Don Walsh landed on its bottom in a primitive submarine, the Trieste, and looked out their tiny porthole for a few minutes. They saw a fish, or maybe it was a sea cucumber. There is life everywhere in the ocean, on every patch of ground, in every ounce of water.
The deep seafloor is perfectly dark—sunlight is completely extinguished at a depth of 3,000 feet—and so it has no plants. Life there is sustained by the intermittent rain of dead organic matter from the surface waters. In places like the North Atlantic, where plankton bloom lushly in the spring, oceanographers find patches of green stuff on the ocean bed, a mile or two below. Sea cucumbers, one of the most common deep-sea animals, crawl through the stuff and vacuum it up. When a fish corpse reaches the bottom, every bit of flesh and bone is slowly scavenged by eel-like hagfish, starfish, and swarms of tiny crustaceans called amphipods. Even where the food is not so rich, the seafloor is not lifeless; everywhere it is churned by bristle worms and nematodes and pill-bug-like isopods. Life at the bottom may be sparse, but it is thorough. Every particle of mud passes through a worm gut several times at least.
For more than a century, after deep-sea studies got going in earnest in the 1870s with the round-the-world expedition of the British ship Challenger, biologists thought that was all there was to it. Then in 1977, two geochemists—Jack Corliss and John Edmond, diving in the submersible Alvin—discovered the first hydrothermal vent, or volcanic hot spring, on the ocean floor. They saw an astonishing scene around the vent. Clustered there, on the midocean ridge near the Galápagos Islands, were giant clams and mussels and six-foot-long tube worms, anchored to the ground and sticking upright. The tubes were white as ivory, with scarlet plumes at their tips that retracted as the sub approached. None of these species had ever been documented before.
Scientists estimate that there were up to 10 million animal species living on the ocean floor. If so, the deep was as diverse as the tropical rain forest.
The strange organisms of the Galápagos rift turned out to be a whole new type of ecosystem. The base of their food chain was not plants that captured the energy of the sun but chemosynthetic bacteria that captured the energy of the volcano. Similar hydrothermal vent communities were eventually found at dozens of other points on the midocean ridge. Biologists, including some who had never thought much about the deep before, descended on them with fascination—and relief. It didn’t take much work to convince the public and the funding agencies that these weird beasts were worth studying, so out-of-reach money suddenly became available. But in the ensuing rush, it was easy to forget that there was still a vast, cold, unknown ocean out there.
Fred Grassle never forgot. He had been one of the first scientists to get a good look at the Galápagos hot springs. A biologist from Rutgers University in New Jersey specializing in polychaetes—tiny caterpillar-like things, also known as bristle worms—he found himself staring out Alvin’s porthole at tube worms almost as tall as he was. He was as amazed as anyone, but he soon went back to the larger problem of studying all the rest of the ocean. In the 1980s, he and his colleague Nancy Maciolek of Battelle Ocean Sciences in Massachusetts used a simple device called a box corer to collect undisturbed square-foot samples of seafloor mud. Judging from how many new species they found each time they lowered their device 7,000 feet onto the continental slope off New Jersey, Grassle and Maciolek estimated that there were up to 10 million animal species living on the ocean floor. If so, the deep was as diverse as the tropical rain forest.