- last post: 01.01.0001 12:00 AM PDT
Probably some sort of Stasis for the Flood, which the covenant must have broken and released the flood from.
Addition: The stasis idea also makes sense because the Halos are meant to starve the flood, so the Forerunners wanted to preserve some of the flood, and would need a way to keep the flood species from being extinct, so that liquid must have kept them from "starving".
[Edited on 1/27/2007]