Steampunk

Via Warren Ellis, a true story very much like something out of his book Planetary.

Colonel John Blashford-Snell discovered the half-submerged, cast-iron wreck off the coast of Panama while searching for ancient ruins.

She was built in 1864 by a visionary craftsman, Julius Kroehl, for the Union forces during the American Civil War. But the boat, called Explorer, was never used in the conflict and was subsequently taken to Panama where she was used to harvest pearls.

She was ideal for this purpose because of a unique lock-out system, identical to the one in the Nautilus from [Jules] Verne’s book, [Twenty Thousand Leagues Under the Sea] published in 1870.

The lock-out system is a reversible air-lock that enables submariners to leave the vessel, harvest pearls from the sea-bed, then return to the submarine. Like Explorer, Nautilus was also used to gather items from the seabed.

Gotta love the name "Colonel John Blashford-Snell." Quoth he,

It had a conning tower and I felt as if Captain Nemo should be in it at the controls.

Oh yeah.