Hydra (code)
From Halopedia, the Halo wiki
Hydra was a UNSC general emergency code. It was almost never used due to its content. The Hydra code word signifies imminent threat from biological weapons of mass destruction.
Cortana used slipspace to declare code Hydra after the discovery of Installation 05 and its escaped Flood the threat came from both the potential for the Halo's activation and the loose Flood that posed a significant threat to all life.
