Karl Stanley's homemade submarine has sprung a leak. It's a discovery both fortuitous and disconcerting. Fortuitous because I notice it as we bob on the surface of the placid Caribbean Sea, just a few hundred feet off the Honduran island of Roatán. Disconcerting because it's 8:30 p.m. and we're about to spend the night 1,600 feet down searching for the six-gill shark, an enigmatic 15-foot, 1,300-pound predator that patrols these depths.

"That's just the O-ring," Stanley reassures me as water wells up in the window and drips to the floor. Apparently the rubber washer meant to seal the window separating us from watery doom is feeling a touch rebellious. "It doesn't have enough compression on it. Hopefully it will fix itself under pressure." Stanley is a problem solver, and he casually throws me a towel to wipe up the moisture, more concerned about the corrosive properties of saltwater than the possibility of a catastrophic breach.

The words homemade and submarine aren't commonly paired, but Stanley, a 34-year-old self-taught engineer, has built two DIY subs, safely logging more than 1,000 dives. Still, sitting in Idabel, the cramped three-person craft he built on a shoestring, I can't help but recall that a faulty O-ring caused the space shuttle Challenger, with its NASA Ph.D.'s and multi-billion-dollar budget, to blow up. Stanley turns a handle, filling the ballast tanks with water, and we begin to sink into unexplored darkness.

Our slow descent accelerates into free fall, and bioluminescent plankton bounce off the submarine, exploding in a blizzard of light. At 100 feet the leak seals, as Stanley predicted, and I feel better about the prospect of finding one of the sharks, which we hope will be attracted by our gruesome hood ornament, a pig's head tied to the front of the submarine.

We plummet through the photic zone, the fertile band of water shallow enough for the sun's rays to power photosynthesis. To my right sits a very squished Martyna Mierzejewska, a 30-year-old Polish-Canadian dive instructor who plunked down $500 for the privilege of role-playing a canned sardine. Stanley, tall and thin like the coconut trees that line the beach near his home, on Roatán's Half Moon Bay, stands in the turret of the L-shaped submarine, driving; he's been taking paying customers down in his homemade subs for a decade. At 660 feet, we leave the photic zone, crossing the ocean's Mason-Dixon line toward the deep sea, the largest ecosystem on the planet, where life is shackled by a dearth of sunlight. Stanley celebrates crossing the invisible boundary with a game of interspecies Morse code, flashing the sub's lights to stimulate the glowing plankton, which respond by burning brighter.

At 1,600 feet, the deep sea's processes complement one another nicely, acting as both trash compactor and refrigerator. More than 700 pounds per square inch nearly 50 times the pressure at sea level squeezes the little yellow submarine, and the water temperature has dropped from the low eighties to the low forties. This is the netherworld that Stanley affectionately refers to as "my zone."