From Halopedia, the Halo wiki
An antigravity plate, or grav plate, is a small form of anti-gravity technology developed by the United Nations Space Command. As of 2531, they were considered old technology and they never were widespread. Grav plates were designed to be attached to other objects, thereby enabling them to float off the ground. Notably, an antigravity plate was used in one instance by Insurrectionists to incapacitate a group of Spartan-IIs by simulating the force of 10 g's of gravity, which caused their Mjolnir armor's compensatory mechanisms to drastically increase the internal pressure of the suits.
Zenostium plasma is a critical component in the manufacturing of antigravity plates. The flux tube between the gas giant Hephestes and its natural satellite, Nephis, is a source of zenostium that is mined by the Nephis Coalition. Due to the importance of the flux tube to the United Nations Space Command's infrastructure, any threat of civil unrest on the moon or commercial rivalry between the Nephis Coalition and competitors is taken very seriously.
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