From Halopedia, the Halo wiki
Permacrete is a human construction material, likely similar to concrete. Some structures built near the edges of the Rubble's far-flung habitats, including Eddie's In The Rock, were constructed by drilling holes into the sides of the habits and then hastily equipping them with permacrete and grating inside. At least one structure on Trevelyan had a basement made out of a one-metre-thick layer of permacrete, and appeared to serve as a bomb shelter.
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