Sentinel wall
From Halopedia, the Halo wiki
Summary
The Sentinel Wall is a massive wall surrounding the the Quarantine Zone around the Library of Installation 05. The Wall appears to have been designed to keep the outbreak of Flood around Delta Halo's Library contained within the QZ. It is honeycombed with many tunnels for the Sentinels to travel through. Sentinels and Enforcers keep the Flood contained, while Constructors keep repairing the damage that the Flood inflict on the Wall. It is unknown if such a structure exists on other Halos.
