From Halopedia, the Halo wiki
Skycrypts consist of a small, circular building located atop a long, thin pole or chain. Four fin-like structures slowly rotate around the pole directly beneath the main chamber. Given the central pole's apparent lack of structural support, the spires may employ anti-gravity technology to remain aloft.
Skycrypts were generally suspended above ice worlds, and considered virtually inescapable to their detainees. However, they offer little defence against external threats. At least fourteen of these structures were located on an unidentified, icy world - with at least one suffering a containment failure.
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