With the decline of seagoing navies in the late 21st century, the submarine all but vanished from military inven-tories. It has made a comeback in recent years, however, because of changes in military defense tactics.
         As 'Mech combat became battles of maneuver and position rather than of firepower, any commander who could disrupt or destroy his enemy command, control, and communication (C3) centers could win a quick, bloodless victory. Consequently, the owners of those C' complexes began to heavily protect and guard them, as the centers had become prime targets for attack. No matter where the C' complexes were located, they immediately drew the fire of every available AeroSpace Fighter, DropShip, and 'Mech in the area. It became a cliche of war that a fixed command post could not survive the first few hours of an invasion. Another proven tenet is that a commander could not control a planetary battle from a mobile command post as effec-tively as from a fixed installation.
         Davion has attempted to avoid this problem by placing many of its C3 complexes underwater. Using Extremely Low Frequency radios, the command posts can effectively control the land battle, while remaining invulnerable to AeroSpace Fighter and DropShip attacks. Only 'Mechs, which can walk along river and sea bottoms, are able to attack the underwater command posts, but this tends to degrade their offensive capabilities. To provide a defense against such underwater attacks, Davion reintroduced the submarine.

         The Neptune was first produced in 2950 by the Galtor Naval Yards, a builder of commercial ships. Even though the vessel displaces 100 tons, the six-man crew of a Neptune are cramped because of the bulk and complexity of its machinery.
         The Neptune is an outstanding weapon. Its Doorman Naval 140 power plant allows for maximum surface or submerged speeds of 54 kph. While on the surface, the Doorman acts like a diesel engine, using hydrocarbon fuel and burning it with the oxygen in the atmosphere. A snorkel allows the diesel to be used while submerged up to a depth of twelve meters. Below twelve meters, the Doorman utilizes a hydrogen peroxide mixture as fuel, with the fuel's bound oxygen as a combustion agent. The ability to use both forms of fuel is an important economy measure for the sub. Diesel fuel is cheaper, and more readily available than the hydrogen peroxide. Moreover, using hydrogen perox-ide causes a progressive degrading of the Doorman's performance over time, accelerating the time between overhauls.
         The Neptune has a double-hull construction. The outer hull is not pressurized, serving only as a framework for the armor plating. The inner hull is the pressure hull. It is rated for dives as deep as 120 meters, though it has been taken down even deeper during emergency situations.
         The Neptune is equipped with various detection and fire control devices. The periscope can extend up to twelve meters, and provides visual observation of surface targets as well as fire control. Both surface and underwater targets can be identified and targeted by the Sonar Sync Tracker, which can function as either a passive or active sonar targeting control station.
         The main offensive armament of the Neptune is the Sea Devastator 20 Rack Torpedo system. A navalized version of the Devastator LRM, the Sea Devastator has excellent range and firepower capability. Two Sea Har-vester 6 rack torpedo systems, another navalised version of a land-based missile system, back up the Sea Devastator. As a secondary weapon, the Neptune mounts a Sutel XII large laser. Though the laser's range is degraded when fired underwater, it provides an important means of damaging shore targets when the Neptune surfaces.

         During the attack on New Ivaarsen in 3021, a Dragon lance of House Kurita's Fifth Galedon Regulars was ordered to destroy the Davion underwater command post, located three kilometers offshore. The Dragons waded into the surf and headed out toward the Davion C3 facility.
         About two kilometers from their objective and 50 meters below the surface of the water, the Dragons started to lose sight of one another. After closing back into visual range, they resumed plodding along. The going was slow, and occasionally a Dragon would slip and slowly settle face-first into the bottom muck, throwing up clouds of obscuring debris. Suddenly, a cluster of torpedoes hit the far-left Dragon. Though none of the shots penetrated, the 'Mech's left arm had sprung a leak and quickly filled with sea water, becoming useless.
         The Davion Neptune let loose five more salvos of Sea Devastators at their attackers. This resulted in the torso of a second Dragon becoming flooded, taking its rear laser out of the fight. The Neptune came closer then and started to pepper the Dragons with Sea Harvester torpedoes. The Dragons could not effectively reply. Their autocannon and LRMs were useless underwater, but the Neptune stayed out of range of their medium lasers.
         The Kurita lance commander decided to withdraw. The Dragons plodded slowly back through the muck, harrassed by the Neptune. One Dragon was lost when its center torso became flooded and its fusion reactor quenched by the onrushing sea. The other three Dragons managed to emerge from the sea, however, battered and nearly destroyed. The Davion C3 complex remained in action throughout the battle for New Ivaarsen, coordinating the planetary defenders and contributing to the eventual defeat of the Combine forces.