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Much of the exploration focuses on the West Coast — offshore from California to Canada — because a long volcanic gash fairly close to shore makes scientific visits there relatively easy. The National Science Foundation has financed much of the work, along with the National Oceanic and Atmospheric Administration.

Five years ago, in a first, scientists off Vancouver Island raised from the depths parts of four rocky vent chimneys, two dead and two live ones spewing hot smoke rich in chemicals and microbes. Dark and rough, they were up to seven feet tall and weighed up to two tons, the hot ones teeming with worms, sea spiders and limpets.

In the June issue of Applied and Environmental Microbiology, the scientists, including Dr. Baross as well as Matthew O. Schrenk, Dr. Deborah S. Kelley and Dr. John R. Delaney, all of the University of Washington, reported the dissection of a chimney that had been venting fluids of 575 degrees. Despite the temperature, it was riddled with signs of life.

"Direct microscopic observation indicated that micro-organisms were attached to mineral surfaces throughout the structure," they wrote, adding that the discovery suggested that further research would expand "the known upper temperature limits of life."

A different census focused on a volcanic gash off Oregon that erupted in 1998, 1999 and 2000, the outbursts monitored by undersea microphones. Each time, the scientists took samples more than a mile down. Such eruptions are windfalls for biologists since not only molten rock but large volumes of hot, microbe-rich water spew forth. The huge clouds of life — thought to originate deep within the cracks, fissures and pores of the rocky seabed — allow experts to glimpse a normally invisible world.

Julie A. Huber, Dr. David A. Butterfield and Dr. Baross, all of the University of Washington, reported their census of microbes up to third of a mile down in the April issue of Microbiology Ecology, published by the Federation of European Microbiological Societies.

They said that even at the greatest depths, under crushing pressures, the rocky seabed was composed of about 30 percent open pores, giving it plenty of living space for diminutive organisms.

The scientists zeroed in on the raw genetic material of the collected microbes, thus finding more than methods of culturing them with special foods could ever discern. (The science of what hyperthermophiles like to eat and breathe is still young.)

To the scientists' surprise, they found a huge diversity of organisms whose composition swung wildly over time. The 1998 eruption produced 35 species of bacteria, compared with 37 and 57 from 1999 and 2000.

But the numbers of archaea — ancient organisms often found in hot places like those thought to exist on the ancient earth — went in the opposite direction, declining from 63 to 60 to 52, according to paper by the same authors in the April 2002 issue of Applied and Environmental Microbiology.

The reason behind the swings is still murky. "We're straining to understand better how these systems work," Dr. Baross said in an interview. "It's a very complicated puzzle. Until a couple of years ago, we had no pieces. Now, to some extent, we're starting to put the puzzle together."

Three years ago, scientists told of finding fossil microbes that lived near vents formed 3.2 billion years ago, confirming that hyperthermophiles were among earth's earliest inhabitants. That discovery has quickened the search for descendants of primordial vent life.

Biologists say the recent discovery of the extremely high-temperature, iron-breathing organism by the University of Massachusetts scientists, who included Dr. Kazem Kashefi, suggests that the dark biosphere runs deeper and hotter than previously documented. And sulfur, they add, may turn out to play a smaller role than previously believed. The iron finding is reported in the Aug. 15 issue of Science.

Dr. Lovley and Dr. Kashefi are betting that the common metal (the earth's most abundant element) will prove important. Its transformations, they wrote, "may have been the first form of microbial respiration as life evolved on a hot, early earth."