Oxygen was kept low — 4.3 percent instead of the normal 19 percent — because the higher amount would be toxic to people at these pressures. In fact, the whole atmosphere was regulated with unusual contents. Nitrogen was kept low at 18 per-cent because it could act as a narcotic at higher levels, and the rest of the air aboard was helium, causing the crew to make duck-like sounds when they talked. The crew was medically monitored to see how they reacted to working at ocean depths.

Crews in Shifts.

The crew could leave Sealab in wet suits, but they could not stay out long because of the cold temperatures. They were to perform certain tasks outside the lab, though, to see how well they functioned. The Sealab II expedition included former astronaut Scott Carpenter leading a ten-man crew off La Jolla, California. There were actually...