Foundry (location)
From Halopedia, the Halo wiki
The Core[1] of the Ark is a planet or planetoid located in the "hole" at the heart of Installation 00. Due to its heavily scarred appearence, it appears to have been extensively mined for resources, most likely to support the construction of the Ark and potential replacement Halos. It can be seen in several levels on the game Halo 3, most notably The Covenant and Halo.
Structure
In 2553, the Core appears to be approximately one third of Earth's dimensions, with a diameter of roughly 4255 kilometres (by comparing t). It could have originally been substantially larger, since the gap in which it is located appears able to accomodate an object at least twice its diameter. Orangey-red colouration can be observed in the fissures which dot the Core, indicating that it may have a molten internal structure.
The Core is partially obscured by dense violet clouds. Although no evidence as to their nature exists, it is highly likely that these clouds are actually swarms of nanomachines, tasked with the construction of new installations. This type of bottom-up manufacture would certainly be within the technological capability of the Forerunners, and it is difficult to imagine how an entire Halo (specifically Installation 04, Mark II) could be fabricated in a matter of months[2] by any other method.
The Core was probably originally located within the Milky Way galaxy, but was moved by the Forerunners. However, it is possible that it was an extragalactic "rogue" planet; not gravitationally bound to any stellar system.
Defences
Along with the massive Sentinel population found around the Ark, the Core has an extra, albeit passive defence. It is equipped with a barrier shield which can be raised to protect both the Core and the Foundry (the industrial infrastructure which supports the creation of replacement Halos) from attack by Flood. At least one superluminal communications array (read: faster-than-light radio) is also included within the barrier's confines.
The Covenant Prophet of Truth raised this barrier in 2552, either intentionally or accidentally. A small section of it is subsequently lowered by a joint UNSC-Covenant Separatist assault force in order to allow access to the Citadel.
