Photos: Underground at the Titan Missile Museum

To launch the missile, the propellant mixture of fuel and oxidizer had to be mixed from four separate valves. To ensure that the missile was not launched except under explicit presidential order, crew members would have to enter a six-digit code that would, if entered properly, unlock a butterfly valve that would allow the fourth valve to release its mixture and, thus, launch the missile. Crew members had six tries to get the code right. If they didn't get it right after six tries, the system would automatically shut down.