Alvin, the US’s only manned research submarine that can explore the sea floor, is back in service after a three-year, $41 million upgrade. This spring it was used in the Gulf of Mexico to assess effects on marine life of the 2010 Deepwater Horizon oil spill.

The titanium personnel sphere is now 18% larger—but with an inside diameter of 2 m, it’s still close quarters for a pilot and two scientists on a typical 8- to 10-hour dive. The new, 8-cm-thick sphere has five windows, two more than before. Alvin also has more and better cameras, provides better lighting, and can carry more instruments down and more samples up.

Most of the new components are designed to withstand dives down to 6500 m. But extending Alvin’s depth rating beyond 4500 m would require powerful, fire-safe batteries; a new variable ballast system; and funding. “At 4500 meters you get 68% of the global sea floor. At 6500 meters, you get 98%,” says Daniel Fornari, a geophysicist at Woods Hole Oceanographic Institution. The US Navy owns Alvin, WHOI operates it, and NSF funded most of the upgrade. (The top photo shows Alvin as divers prepare it to be lifted from the water after a test dive. In the bottom photo, a Styrofoam cup shrunken by pressure when Alvin carried it to the sea bottom stands alongside one that stayed on Earth’s surface.)

In its 50 years, Alvin has had a significant impact on deep-sea research—and revolutionized deep-sea biology by discovering hydrothermal vents, says Fornari. Before the latest upgrade, the sub had made 4664 dives. They were for studies in biology (about 38% of dives), geology and geophysics (32%), and chemistry (13%); the other 17% of dives were for engineering and equipment tests, training, and education.

Peter Girguis, a Harvard University deep-sea microbiologist, designed an underwater mass spectrometer that he takes down on Alvin. “I care about measuring chemicals in relationship to the organisms I am studying,” he says. “The biomass in the deep ocean is modest compared to the upper ocean, but the diversity of life in the deep sea is way larger. Understanding microbes is key if we want to have a sense of how humans affect the biosphere—acid rain, ozone hole, climate change.”

Alvin is also used to measure geophysical properties of the sea floor and ocean crust. Localized gravity measurements, for example, can distinguish sub-sea-floor areas of dense rocks from porous lava flows or sediments and can identify zones of mineralization created at hydrothermal vents. “Underwater hot springs have high amounts of copper, gold, and even platinum,” says Girguis. “That’s spurring a renewed worldwide interest in deep-sea mining.” And magnetic field measurements can map reversals of Earth’s magnetic field that are recorded in volcanic rocks; researchers use those measurements in calculations of sea-floor spreading and crustal accretion.

Mike Perfit of the University of Florida recalls a series of dives in 1991 to a section of the mid-ocean ridge southwest of Mexico. The team could not find features that had been previously mapped with a deep-sea camera towed from the sea surface. “Fresh black glassy basalt covered everything and white bacterial floc that looked like snow [was] coming out of cracks and holes in the sea floor,” he says. Postdive analysis of short-lived radioisotopes in lava samples confirmed that they had seen a volcanic eruption. After studying the site for more than a decade, Perfit and colleagues were surprised to discover that such eruptions occur on decadal time scales, rather than hundreds to thousands of years as had been thought.

Often observations with autonomous or tethered robotic sea-bottom vehicles complement research with Alvin. With robotic vehicles, says Susan Humphris, a WHOI geochemist and the principal investigator for the Alvin upgrade, “you are looking at a TV screen, and you don’t get the context of the rocks. You don’t understand the relation of things to each other in the same way as you do with the human eye.”

Alvin is more responsive than tethered robotic vehicles, says Ken Macdonald, a University of California, Santa Barbara, geophysicist who has been on about 50 dives. “It’s more efficient for taking samples and deploying instruments.” And, he says, “there is the exhilaration. I can’t emphasize enough how important that is. Being down there is profound.”