The explorers who set one of the last meaningful records on earth.

Sea level—perpetual flux. There is a micromillimetre on the surface of the ocean that moves between sea and sky and is simultaneously both and neither. Every known life-form exists in relation to this layer. Above it, the world of land, air, sunlight, and lungs. Below it, the world of water, depth, and pressure. The deeper you go, the darker, the more hostile, the less familiar, the less measured, the less known.

A splash in the South Pacific, last June, marked a historic breach of that world. A crane lowered a small white submersible off the back of a ship and plonked it in the water. For a moment, it bobbed quietly on the surface, its buoyancy calibrated to the weight of the pilot, its only occupant. Then he flipped a switch, and the submarine emitted a frantic, high-pitched whirr. Electric pumps sucked seawater into an empty chamber, weighing the vessel down. The surface frothed as the water poured in—then silence, as the top of the submersible dipped below the waterline, and the ocean absorbed it….