Two short blasts are sounded on the diving alarm, the second blast is the signal to start the dive. In the movie the diving signal and verbal order were given three times instead of twice. Shut outboard and inboard engine exhaust valves, shut hull ventilation supply and exhaust valves, shut inboard engine air induction flappers, shut conning tower hatch. Open bow buoyancy vents and all main ballast tank vents, except the group or tank designated to be kept shut until pressure in the ship indicates that all hull openings are shut. Use stern planes to control the angle of the ship.
Air is bled into the ship when green lights show that all hull openings are shut.
Watertight integrity is assured when the internal air pressure remains constant. At 15 feet short of the desired depth, blow the negative tank, shut its flood valve, and vent the tank. Diving officer reports to conning officer when trim is satisfactory.