Hydrostatic gel
From Halopedia, the Halo wiki
Hydrostatic Gel is a component of the MJOLNIR armor created by the UNSC and its Spartans. It is a bluish semiliquid that is actually located inside the armor, where it makes direct contact with the wearer's skin.
Hydrostatic Gel serves several purposes; first, it conforms to the wearer's shape and body temperature, keeping the wearer from getting hot or cold. second, the Gel can be pressurized to various levels, allowing the wearer an amount of cusioning during hard falls. This way, a wearer can survive virtually any height of fall as long as they pressurize the armor.
There is an emergency exhaust port in the armor that releases Hydrostatic gel when the suit becomes too hot.
This may explain why the Master Chief can fall from any height in Halo 2 with no damage.
