In the world of Halo, there are 7 installations (01-07) spread throughout the galaxy. So far we know that they were built many millenia ago by an alien civilization known as the Forerunners. Their purpose? To contain the parasitic Flood and in any catastrophic emergency, to activate and wipe out all sentinent life in a 25,000 light year radius (when all 7 activate the whole galaxy will be devoid of sentinent life) in order to deprive the Flood of food and means of spreading. Halo: Combat Evolved involves only one of the total seven, Halo Installation 04.