Answer:
To enter or exit (or escape from) a submarine you must locate a hatch in the pressure hull. There is usually a hatch near the bow, the stern, and amidships, and each is set atop a feature called an escape trunk. The escape trunk is a small "compartment" with an inner hatch, and it is set up to provide a way to "lock out" a group of submariners that is making an emergency exit under water. A torpedo tube might also be used to escape.