The thing about the mud hoisted from the bottom of the Black Sea in the summer of 2001—the thing that surprised and delighted the researchers aboard the Professor Logachev—was that there was hardly any mud at all. They were 75 miles west of the Crimean city of Sevastopol, 750 feet above an undersea slope along which sediment from the Dnieper River cascades down into the depths. With a set of giant steel claws guided by a video camera, they had taken a one-ton bite out of that slope and dumped the goopy mess on deck. It stank. That didn’t surprise anyone—seafloor mud often contains hydrogen sulfide, which smells like rotten eggs. But they were struck by what else was in the sample: “Nearly one ton of biological material,” says Walter Michaelis, a biogeochemist at the University of Hamburg in Germany, who led the expedition. “No sediment. No carbonates. It was a cubic meter of bacteria!”

Photograph by Jonathan Kantor

A few days later, Hamburg geologist Richard Seifert got a look at the seafloor where the bacteria came from. Diving in a little German submarine called Jago, peering through an acrylic porthole several inches thick, he saw a fog of floating particles. Then, looming out of the dark and into the sub’s tight halo of floodlight, through curtains of rising bubbles that made it seem as if the sub were driving through champagne, Seifert saw chimneys—black, knobby spires, the tallest rising more than 13 feet off the seafloor. Chimneys made by erupting volcanic minerals are common at hot springs in mid-ocean ridges. But this was not a hot spring, and these chimneys were not built of volcanic rock. The pilot prodded one with Jago’s hydraulic arm. It was soft, like flesh. He knocked one over, felling it as if it were a tree to reveal its cross section. Under a black outer layer there was a thick layer of pink and a core that was harder and greenish gray.


The chimney had been made entirely by single-celled microbes. The microbes formed the outer layers; the hard core was a carbonate mineral they had secreted. What Seifert and his colleagues had discovered was an outcrop—of life. The evidence is now clear that far below the sea, and far below the floor of the sea, in sediments all over the world, microbes live to astonishing depths—the record so far is half a mile—and in astonishing numbers. The deepest of the microbes make methane, which the ones in shallower sediments consume. To all of them, oxygen is poison. They are relics of an early period in Earth’s history when methane was abundant and green plants had not yet given the planet oxygen. “Maybe the early Earth was all covered with blackish, pinkish slime,” says Antje Boetius, a biogeochemist and geomicrobiologist at the International University in Bremen, Germany.

Today these ancient organisms have been pushed into obscure niches where oxygen does not penetrate. Mostly that means below the seafloor—except in a few special places like the Black Sea. With only the narrow Bosporus as an outlet, the Black Sea is seldom flushed, and the oxygen in its deep water, below 600 feet or so, has long since been depleted. No fish live at that depth. But microbes do, thriving on methane that bubbles up from below. The methane is made by the microbes’ deeper cousins, and they are numerous.

The total mass of microbes living beneath the seafloor has been estimated at as much as a third of all the living stuff on the planet. The total amount of methane made by these microbes is probably greater than the mass of all known reserves of coal, gas, and oil. Methane is a potent greenhouse gas, and huge belches of microbial methane from deep reservoirs, where it resides mostly as frozen methane hydrate, have been linked to rapid changes in Earth’s climate. They may have helped pull the planet out of recent ice ages, and they almost certainly helped end the Paleocene Epoch 55 million years ago with an intense burst of global warming. Nor are the potential impacts of deep-sea methane limited to climate. Blasts of it have been linked, in respectable journals, to mass extinctions, to undersea landslides that caused ocean-crossing tsunamis, and even to the mysterious disappearance of ships at sea.

All that may seem a lot of action to attribute to mud. But that’s precisely the essence of what researchers have been finding lately: Seafloor mud is alive, and it is powerful. It’s the whale we managed not to notice until now.