UNSC Engineering Corps
From Halopedia, the Halo wiki
UNSC Engineering Corps or ENGCORP for short, is a branch of the UNSC Navy devoted to surveying viable planets for colonization or terraforming. Engineers from ENGCORP visit these planets, and make field reports documenting features of the planet, suitability for colonization, and other details that would be necessary for a possible colonization. These reports are then published for command to give the "O.K." signal.
The Engineering Corps is Credited for discovering Onyx, the massive Forerunner Shield World that housed the training for the SPARTAN-IIIs.
At some point during the Human-Covenant War, Marines from the Engineering Corps would make a last stand defending the civilians of the colony Algolis.
