It was a perfect spring day to be out on San Francisco Bay, but Chris Welsh, a burly six-foot-three competitive sailor, was cooped up inside a dimly lit workshop, tinkering with a one-man submarine that looked like a boxy glider with stubby wings. The shop, perched above a wharf at Point Richmond, is the home of Hawkes Ocean Technologies. The ship, the Virgin Oceanic, is the result of a collaboration between Welsh (who will pilot the sub), the British engineer Graham Hawkes, and Sir Richard Branson. They’ve got high—or, rather, low—hopes for their project.

“It’s not built for speed,” says Welsh, 48. “It’s built for depth.”

Specifically, the Virgin Oceanic is designed to reach 36,000 feet—the bottom of the ocean. And Welsh isn’t alone in his quest.

This summer, three crews are on separate missions to build the next big thing in ocean exploration: a manned submarine that can scoot around the Mariana Trench, the deepest place on the planet. In addition to Welsh’s Virgin team, Avatar director James Cameron is financing a sub, and Florida-based Triton Submarines is trying to produce a commercial model, the Triton 36,000, that can take clients to earth’s final frontier for as much as $250,000 a head.

A crew has been to the trench before, but not since 1960. That’s when Swiss oceanographer and engineer Jacques Piccard, along with 28-year-old U.S. Navy lieutenant Don Walsh, piloted a homemade, 150-ton steel bubble called the Trieste to the floor of the trench. Maybe it’s because he’s the only man alive today who’s been there, but Walsh, now 79 and the honorary president of the Manhattan-based Explorers Club, doesn’t see the point. “Usually, if there’s a race, it implies some kind of prize,” he says dismissively. “What’s anyone going to win, the right to be second?”

But there are broader implications here. These new submarines are as different from the Trieste as a stealth fighter is from a hot-air balloon. The Trieste operated in the manner of most deep-sea submersibles: loaded with weights that were later jettisoned and equipped with flotation devices, it traveled mainly in two directions—down and then up. Though it had thrusters that allowed for horizontal movement, Walsh and Piccard spent their 20 minutes at the bottom in one spot, surrounded by their own silt cloud, before surfacing with a cracked viewing window. The next generation of explorers hopes to have a look around.

While no single technological breakthrough is rekindling our interest in exploring the Mariana Trench, a series of incremental advances—including pressure-resistant syntactic foam, lightweight lithium batteries, and advanced ceramic viewing ports—have led all three groups to the same conclusion: we’re on the cusp of designing subs that can make regular trips to the depths.

That’s provided everything goes according to plan. Even the slightest engineering mishap could have dire consequences under deepwater pressures reaching 16,000 pounds per square inch. During ­laboratory testing, for example, the Virgin Oceanic’s borosilicate viewing bubble cracked under just 2,200 psi. Hawkes thinks he’s fixed that problem, which he traced to the ship’s joints shrinking at different rates. Still, a malfunction like that at the bottom of the ocean could mean instant death for anyone inside.