A Halo Ring is a large installation created by the Forerunners. It was built to destroy the Flood.
When the Forerunners were still around, they were invaded by the Flood. At first, the Forerunners thought they could fight off the Flood, but they couldn't. Instead, they built the Halo Rings, which would destroy all sentient life in the galaxy. Samples of DNA from each species were kept safely, so after the blast, the galaxy could be repopulated.
Each Halo ring has it's own unique landscape and characteristics.
Installation 00 (The Ark)Edit
Visited in Halo 3. The central firing pont for all the Halos, The Ark's landscape includes deserts, forests, oceans, and an Earth-like atmosphere.
Seen in hologram form in Halo 3, this installation's environment is endless desert and has limited atmosphere.
Seen in hologram form in Halo 3, this installation's environment is water and ice-based with rocky outcroppings.
Seen in hologram form in Halo 3, this installation has a red, cracked surface, indicating vast seserts and volcanic acrivity.
Installation 04 (Alpha Halo)Edit
Visited in Halo: Combat Evolved and Halo: Combat Evolved Anniversary, this installation has forests, caves, mountains, blizzard-like deserts, and an Earth-like atmosphere. Installation 04 was destroyed by Master Chief.
A near-identical yet unfinished replacement for the destroyed Installation 04, this installation's weather control was not yet online. As a result, the majority of the ring visited was only snow. This installation was destroyed by Master Chief.
Installation 05 (Delta Halo)Edit
Visited in Halo 2, this installation's environment mostly consisted of forests, rivers, and ruins. After a massive Flood outbreak, the surface of Installation 05 was glassed.
Seen in hologram form in Halo 3, this installation is colpletely barren and rocky.
First feen in hologram horm in Halo 3 and later as the primary setting for the novel Halo: Primordium, this installation's environment is mostly desert with a harsh atmosphere.
In Halo: Combat Evolved Anniversary, 343 Guilty Spark makes note of, in a terminal, that there were originally 12 Halo rings, but 5 were destroyed (leaving 7 and the Ark).
On the ringEdit
On the inner side of the ring is a large structure built to simulate a planet. It has continents and oceans. There are also simulated weather conditions like rain, snow, and so on. Somewhere on this part of the ring is the control room, where the ring can be fired.