Marlow’s hunt for extreme life has required some extreme living. During his time studying geobiology in Victoria Orphan’s lab at Caltech and since, Marlow’s research interests have taken him from the boiling Marum Crater lava lake, more than 1,000 feet down inside an active volcano on a small island in the South Pacific, to the cold, dark depths of Hydrate Ridge off the coast of Oregon. On that expedition, the research vessel Atlantis ferried a group of scientists some 60 miles from the shoreline, and the U.S. Navy’s three-person submersible then carried Marlow and his colleagues another half mile straight down into the Pacific Ocean. “Everything moves more slowly down there,” Marlow says, describing the languid attention the sea animals paid to this new man-made creature in their midst. The sub’s headlights illuminated only a small portion of inky ocean. “You know you aren’t getting the full picture,” he says, “but your eyes are always drawn toward the edge of the light, trying to resolve what’s just beyond your grasp and figure out how it all fits together.”

What Marlow has been trying to figure out is how certain microorganisms inhabit deep sea cold seeps—fissures in the ocean floor that leak methane and other gases. The microorganisms Marlow is most interested in live under about three centimeters of ocean mud and consume methane, preventing the powerful greenhouse gas from entering the atmosphere. On the seafloor, Marlow and his fellow scientists look for the telltale sign of the methane-eating microorganisms, a white and orange carpet of other microbes attracted to their waste products. That’s where the sub’s pilot collects rocks and sediments. Back on the ship, it’s easy to tell if the science team has found what it was seeking: The microorganisms reek of sulfide—that same rotten egg smell Marlow encountered on his childhood trip to Yellowstone.