The Catalog

The Catalog was a Forerunner installation, built for data storage.

Background
It is not known exactly when the Catalog was constructed, but was before the activation of the Halo installations in approximately 100'000 B.C. Built primarily for data and information storage, the Catalog held all Forerunner knowledge and research, including locations of various star systems, technological advances and even information on the Parasite.

It's location still remains a mystery, and it is unknown as to whether it still exists, or was destroyed during the Forerunner-Flood war. It is also suggested that Forerunner AI's were created at the Catalog, before being sent to their required area.

The Catalog itself is thought to be a space space station, although of unknown shape or size. However, evidence given by various AI's of Forerunner origin suggests it is atleast the size of Lunar.