The Ice Fields is a system of mainly frozen planetary bodies. The majority of the frozen bodies are composed of gasses and ice. These Ice Giants are commonly harvested for their valuable Ice Fragments. The Ice Fragments contain all the basic requirements for initial terreforming. They only need to be melted to work, but it is easier said than done. The ice is always refreezing itself in a matter of seconds. It needs a high powered heating core placed in the center of the Ice Fragment to melt it, though often the cores freeze before activating.
There are 27 Ice Giants in the Ice Fields. Each one producing new ice by the second. The planet Tanis is highly valued as its ice is by far the most valuable. Many non UNSC affiliated humans have fought over the rights to the planet and its substances. Not only does Tanis have the best Ice available in the system, but it also has a small habitable moon orbiting it that is constantly fed Ice Fragments that are blown off of Tanis from the gas build up beneath the ice.