Possibly AU crossover between Halo and Subnautica. In 2186, the Aurora, a ship of unidentified class built by Alterra, and the Idolageian, a Phoenix-class vessel built by Misriah Shipyards, are sent to Planet 4546-B on a routine terraforming mission. When the Aurora is strcuk by an unknown projectile, the mission is changed. Humanity is about to wake up.

Spartan-IIIs B312, B318, B332, and A117 have just arrived on planet Reach, the heart of the UNSC's naval might, to join up with Spartan Team NOBLE after the death of a NOBLE squad member - just in-time for the Fall of Reach. These four Spartans will insure the survival of humanity through failure, tragedy, and death, before they themselves fall. Will Reach survive? AU