From Halopedia, the Halo wiki
The Foundry, also known as the Forge, is a colossal manufacturing facility located at the heart of an Ark installation, used to construct Halo ringworlds. Both Installation 00 and its older counterpart, the greater Ark, possessed one. Installation 00's Foundry was home to a scarred planetoid that had been extensively mined by Forerunner machines, either to support the construction of the Ark or for potential replacement Halo installations.
The greater Ark's Foundry produced the original twelve Halos, each 30,000 kilometers in diameter. However, it was incapable of constructing the smaller, more powerful Halo installations designed as an improvement over the original rings, necessitating the construction of a second Ark.
The planet in the center of Installation 00's Foundry was partially obscured by dense violet clouds. Although no evidence as to their nature exists, it is possible that these clouds are, in fact, armies of Retriever Sentinels, tasked with the construction of new installations. This type of bottom-up manufacturing would certainly be within the technological capability of the Forerunners, and it is difficult to imagine how a construct like Installation 08 could be fabricated in less than three months by any other method.
The planetoid at the center of the Foundry was originally located within the Milky Way galaxy, but was moved through slipspace by the Forerunners in order to be mined. It is implied that many planetoids had been exhausted before mining started on the moon present in 2552.
In late 2552, the Foundry's central planetoid appeared to be approximately one third of Earth's dimensions, with a diameter of roughly 4,255 kilometers. It could have originally been substantially larger, since the gap in which it is located appears able to accommodate an object at least twice its diameter; although the large gap may have been intentional as to minimize the planetoid's gravitational pull on the Ark's inner edge. Orange-red coloration can be observed in the fissures which dot the planetoid, indicating that it may have a molten internal structure.
The Foundry generates so much heat that the Ark requires multiple cooling stations to keep the installation inhabitable. At least two such systems span a snowy gorge; it is possible that the massive cooling systems cause this icy climate.
Following the destruction of Installation 04, the Foundry began crafting a replacement. During a mission to Installation 00's Cartographer, 343 Guilty Spark appeared to recognize this while looking at a map of the installation, but was prevented from explaining what he had seen. Shortly after the Battle of the Citadel, Installation 08 rose out of the Foundry and into space in an incomplete but nearly-finished state. Upon seeing it, John-117 and Thel 'Vadam realized its purpose and what had happened.
After the destruction of Installation 08, the Foundry began constructing a second replacement for Installation 04, known as Installation 09. By the time of the arrival of the UNSC Spirit of Fire in 2559, the installation was almost complete.
When the Banished arrived on Installation 00, they took over the Foundry to act as one of their bases. In order to destroy the Banished flagship Enduring Conviction, the smart-AI Isabel fired Enduring Conviction's ventral energy projector into the Ark which resulted in the installation's Aggressor Sentinels destroying the carrier. The subsequent launch of Installation 09 opened portals to the ring and the Spirit of Fire's forces were forced to attack the Banished base in the Foundry to stop them.
During the Second Ark Conflict, the almost-complete Halo installation in the Foundry was discovered by Professor Ellen Anders who proposed launching the ring to the Soell system in order to send a distress beacon to alert the UNSC of their situation. Following the destruction of Enduring Conviction, Anders launched the ring from the Foundry and it subsequently entered slipspace en route to the Soell system.
Along with the Sentinel populations found around the Ark, the Foundry has an extra, albeit passive defense. It is equipped with a massive energy barrier, which can be raised through activating barrier towers to protect both the core and the Foundry from a possible Flood attack. The Ark's Citadel, a superluminal communications array and the control center of the entire Halo system, is included within the barrier's confines.
The Prophet of Truth raised this barrier in 2552, either intentionally or accidentally. A small section of it was subsequently lowered by a joint UNSC - Sangheili assault force in order to stop Truth from activating the Halo Array.
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