During our recent tour of the USS John Warner nuclear-powered submarine, we got a chance to see a small compartment known as a "lockout trunk."

"This is actually how we would get SEALs off the ship submerged," Senior Chief Mark Eichenlaub told Business Insider.

"So you would stick a platoon of SEALs in here, 14 guys ... you fill this chamber with water until you match the outer sea pressure. Once the pressure in and outside the ship match, the hatch will lift off open, and they can swim out of a fully filled chamber into open ocean."

Once the chamber is filled with water, matching the pressure inside and out, "there's an internal locking mechanism that would open" the top hatch where SEALs swim out, Senior Chief Darryl Wood told Business Insider.

The SEALs can then swim to retrieve what is known as a special-forces operations box, which would be filled with weapons and needed gear, from the tower.

A member of SEAL Delivery Vehicle Team Two prepares to launch one of the team's SEAL Delivery Vehicles from the back of the Los Angeles-class attack submarine USS Philadelphia on a training exercise. (Navy photo by Chief Photographer's Mate Andrew McKaskle.)

In addition to getting SEALs off the ship, lockout trunks can be used for the entire crew to escape in case the submarine is downed.

This video gives a close-up look at the lockout trunk: