Submarines are stealthed while moving or standing still and invisible to enemy units except for detectors. They are also detectors themselves, capable of detecting hidden enemy units, including enemy subs and land units like Spies. Submarines reveal themselves only when attacking. They become stealthed again when not attacking for a while (roughly 20 seconds). Light Ships and Helicopters have the ability to detect Submarines and are very effective in destroying them.
Submarines' production cost increases progressively with every Shipyard unit on the field or currently in production. The default increase is of 2 Oil for the second unit created, increasing further with every new unit until the cost cap is reached, which for military units is +125% of their original base value.