The Bathysphere is a naval unit unlocked by the Bathysphere technology in Research Era III.
A city must have at least one exploited sea or lake tile where the Bathysphere will spawn upon completion, which is normally designated by a movable Anchor icon in the city screen. If there is a naval army on that tile owned by the owner of the city with room for more units, the Bathysphere will automatically be merged into that army upon completion. If the designated tile is occupied by an army with no more room, an enemy naval army, a transport ship, or a tile of ice shelf during winter, the Bathysphere will spawn on the nearest available exploited sea or lake tile. If there are no available sea or lake tiles, construction of the Bathysphere cannot be completed.
This highly secretive unit can travel almost invisibly beneath the waters.