The HMS Thetis

When the submaringe Thetis made a trial dive in shallow water, it did not know that a malfunctioning torpedo tube would cause it to ram into the seabed below. A miscalculation of the sub's air supply prevented panic among the crewmembers, and an escape pod was sent to the surface to get help. Rescuers worked with the stern sticking out of the water until the sub flipped over and sank entirely. By this time, the 99 passengers had long since suffocated to death.