In a dimly lit passage, line with corrugated metal some 50 feet underground, we donned our dive gear: pulling on our dry suits, shrugging our arms through the harness on our backplate and wing, tugging our hoods down and over our heads, and pulling our gloves on. Fins in hand, we then walked over to a set of stairs - 48 in all - suspsended precariously 75 feet above the surface of the water. We were in a giant concrete tube about 60 feet in diameter and 200 feet tall. Above us, access to the surface was sealed off by a set of 25 ton nuclear blast-resistant steel doors. Below us, 100 feet of inky black but crystal clear water. Carefully, we placed our feet on each step, gripping the handrails for dear life, and made our way down first the wooden steps, which gave way to metal steps, and finally a metal ramp, to a floating dock.