Though the vent field and a sister site nearby were found in spring 2010, details and pictures of new species at the site were released just yesterday.
Located near the Cayman Islands, the hot seafloor vents are nearly 3.1 miles (5 kilometers) deep—about 2,900 feet (880 meters) deeper than the previous record holder.
About 12.4 miles (20 kilometers) away, the expedition also found a second mineral-rich vent field close to a section of seafloor where tectonic plates are separating.
There, unexpectedly, a chunk of mantle is poking through Earth's crust. (The planet's hot, semifluid mantle rock layer is typically contained miles below the planet's thin, hard outer crust.)
Though the mantle area is only half as deep as the deepest vent field, both sites are inhabited by hordes of new species, including sea anemones, fish, and swarms of 1-inch-long (2.5-centimeter-long) shrimp (above) with purple organs that may detect infrared light beaming from the vents.
"We were absolutely stunned to find these vent sites," said marine geochemist Doug Connelly of the U.K.'s National Oceanography Centre and a member of the expedition.
Vents like this "may be far more common than we think they are," he said. "We just may not be looking in all of the right places."
The discoveries follow closely behind another deep-sea expedition just north of Antarctica by the same team. That investigation found warm bastions of life—including new yeti crab species and a ghostly octopus—on the usually near-freezing ocean bottom. (See pictures: "'Lost World' of Odd Species Found Off Antarctica.")
Details of Earth's deepest volcanic vents are described today in the journal Nature Communications.