Escape trunk

An escape trunk is a small compartment on a submarine which provides a means for crew to escape from a downed submarine; it operates on a principle similar to an airlock, in that it allows the transfer of persons or objects between two areas of different pressure.
Principle of operation.
The water pressure on the outer hatch is always greater than the air pressure inside the submarine, which prevents opening the hatch. Only when the pressure inside the escape chamber is equal to the sea pressure can the hatch be opened. Thus the compartment must be sealed off from the interior of the submarine and the pressure inside the chamber must be raised to sea pressure in order to make it possible to open the escape hatch.
DSRV rescue.
The escape trunk can also be used for the rescue of a crew inside a disabled submarine using a Deep Submergence Rescue Vehicle (DSRV). The crew of the DSRV will maneuver the vessel above the upper hatch of the escape trunk and then latch on to the submarine using the built-in eyes on the outer hull of the submarine. Most submarines do not have the built-in eyes and rely on the DSRV establishing a watertight seal on the area surrounding the hatch. Once the sub has settled on the correct location the skirt is pumped out and the hydrostatic pressure on the exterior of the skirt is enough to maintain the seal when the hatches are opened and the transfer undertaken. The crew can then quickly equalize the pressure in the small chamber between the DSRV and the submarine and open their own lower hatch.
On the outside of the upper hatch of the escape trunk is often a shaft connected to the wheel that operates the locking mechanism. The crew of the DSRV can attach a wheel to the shaft and turn the shaft to unlock the hatch, thus providing access to the interior of the submarine. This is a risky operation, as when a submarine has been damaged enough to sink, it is possible there was an ingress of water into the pressure hull raising the internal pressure above the normal 1 bar.