CASTLE Base
From Halopedia, the Halo wiki
Summary
ONI's CASTLE Base is located on the planet Reach, 2,000 meters below the Highland Mountains. Protected by the massive amount of granite, it is virtually bombproof, well defended and impenetrable. CASTLE Base could withstand an 80 megaton nuke without a problem.
It was secretly built to lead into an underground Forerunner srtucture.
