Installation 04

Installation 04 was one of a series of gigantic ring worlds (the Halos from which the series gets the name) apparently left behind by the enigmatic, technologically-advanced, and apparently extinct Forerunners. The Covenant reverentially described Installation 04 as either the First Holy Ring, or the First Sacred Ring. 04 is also known colloquially among Halo fans as Alpha Halo. It is the main setting of the first Halo game, Halo: Combat Evolved. After its destruction at the end of the Battle of Installation 04, The Ark started to make another Installation 04 after the first Installation 04 was destroyed.

Background

The purpose of Installation 04, along with its sister installations, was to preclude and thwart the parasitic lifeforms known as the Flood from consuming the galaxy's sentient life. In the event of a Flood outbreak, one of the seven installations could be triggered to annihilate all sentient life. Once a ring is set off all remaining rings would activate.
Installation 04, until its destruction by the Master Chief, was controlled by the Forerunner artificial intelligence construct (Monitor), 343 Guilty Spark.

Climate

The climate of Installation 04 is very similar to Earth's climate. There have been hurricanes over the oceans on the ring. The weather is, however, extremely unpredictable - rain and snowfall can occur at random, leading some to wonder if the weather is intentionally irregular, or whether there is some indiscernible pattern.

Ancient History

Installation 04 was built by the Forerunners at approximately 98,665 BCE (It was about 101,217 years old in 2552 CE).

Part of the wreckage of Installation 04
The Installation was activated sometime thereafter, and all Forerunner and Flood were killed. Installation 04 lay dormant for millenia, with 343 Guilty Spark and his Sentinels ready to keep the Flood confined in the Flood Containment Facilities and keeping the infrastructure of the Halo in perfect working order.

Destruction

"Halo... It's finished...""No. I think we're just getting started."
— Cortana talking
to the Master Chief after the
destruction of Installation 04.

The Covenant discovered the location of Installation 04, after the Covenant Fleet of Particular Justice followed the single UNSC cruiser, the UNSC Pillar of Autumn, which was fleeing from the Battle of Reach through slipspace. It is assumed that the Covenant knew about the Ring as the Prophets all have a holographic representation of them on their headdresses and Cortana saying previously that she used the coordinates received by the beam in The Fall of Reach. Due to the Covenant's faster Slipspace capabilities they arrived before the Autumn.
The Pillar of Autumn arrived around September 19, 2552 and was promptly shot down by warding Covenant space forces. At around the same time, the Covenant ground forces on 04 accidentally unleashed the Flood from captivity. The ensuing four-day melee that took place all over the ring was called the Battle of Installation 04. Humans, Flood, and Covenant fought massive battles against one another as the Forerunner Constructs tried to control the outbreak by activating the Halo. In the end a single human, SPARTAN-117, detonated the Autumn's fusion reactors which destroyed the Halo.
Only a handful of humans and Covenant survived the battle. 343 Guilty Spark floated through space until he met a group of Covenant on Threshold. Inspired by his words, they began a Heretic rebellion against the greater Covenant. Installation 04's destruction is seen as the first step in the chain of events that led to the Covenant Civil War. The Installation 04 was the first Halo destroyed.
There is now a large field of debris where Installation 04 was, though a few large chunks of the ring remain. Some smaller pieces of the Installation 04 have landed on Basis and others have burnt up in Threshold's atmosphere on entry.

Replacement

Shortly after the destruction of the Halo, the Ark began to automatically construct a new Halo to replace Installation 04. This exact copy of the first Halo ring was nearing completion only a few months after the first ring was destroyed, but was subsequently also destroyed when it was fired before it was ready by the Master Chief and Cortana.

Astronomy

The "Ringworld" of Halo is much smaller than Larry Niven's Ringworld, by a magnitude of 3,000. While the diameter of Niven's ringworld is close to the diameter of Earth's orbital of 300,000,000 km, the 10,000 km diameter of Halo is much closer to the diameter of Earth itself, which is 12,756 km. Ringworld has a star similar to our sun in its center, and Halo is orbiting a planet, but does not encircle it.
Circumstantial evidence from Bungie indicates that Alpha Halo may be located in the Iota Horologii System, only 50 Lightyears from Earth.

Speculation

It is unclear why the gas giant Threshold was specifically chosen to be the planet which Installation 04 orbited, but it is supposed that the large number of Forerunner machines already operational in Threshold (explicitly the network of Gas Mines), may have played a role in their decision. It could also be because it was the only planet in the system that had enough gravity to hold Halo in place.
Another common theory is that it was placed there mathematically in order to cover the whole of the galaxy with the spread of the Halos' main weapon. It is probable that the Forerunner had chosen Threshold as the staging point for constructing Installation 04 in a failsafe to attempt to isolate any major Flood outbreak. It may have been as simple as its proximity to the Gas Mine stations on Threshold, which the Forerunner may have used for power.