The drowned carrier-sized submarine, now revealed as the vacuum pump ramming the ocean into the necropolis — Carl's raid target.
The Akula — dead bugbear treasure-hunter Shamus Chaindrive's enormous submarine, seized by Chris to liberate the water quadrant — is now understood to BE the flooding mechanism. Its sail torn away by a leviathan and its nose jammed into a hole at the base of the Necropolis of Anser, its torpedo tubes vacuum vast amounts of ocean straight into the tomb. Carl's team dives 500 meters to shut it off: entering through the ripped superstructure, they swim the flooded, oil-smelling corridors past mechanical debris and drained airlocks, loot the Bugbear's Delight neighborhood-boss box (killing the sentinel gun outside the bridge), and pull the sub's full map — flooded decks in red, dry sections marked, the fire-control station below the bridge holding the pump switch, escape hatches above (two of three spent).
But the map also reveals Quetzalcoatlus's ghost flickering through the bridge, in and out of the water quadrant entirely.
A vast carrier-sized submarine, more spaceship than boat, slick dark metal stretching into blue gloom, sail torn away, whole hull vibrating as it pumps water; interior flooded, creaking, strewn with mechanical parts and bugbear-metal scrap.
The water quadrant's boss-castle and the pump flooding the necropolis — the object of Carl's underwater raid.