From Halopedia, the Halo wiki
The Knowing was located in a Forerunner facility behind a series of corridors buried underneath Ariel's surface, and filled with traps, such as wall-mounted, motion-activated lasers. Its function was to observe, record, and archive information over the course of the galaxy's history. Presumably, it had been collecting information for the 100,000 years since the disappearance of the Forerunners after activating the Halo Array.
During Operation: FIRESIDE, ODSTs Dutch and Romeo found The Knowing after their stolen Phantom crash-landed in a room containing a group of missing colonists whom the Covenant had been sending through the tunnels to find traps.
Upon entering the Knowing's chamber, it acknowledged their primacy and offered to answer any questions they had. When it was revealed that the Covenant would be able to use the Knowing's information to destroy the human race "down to [their] last DNA strand", the ODSTs decided to tell the Knowing to self-destruct before the Covenant could reach it. Its destruction gave cause for the Covenant to leave Ariel, seeing no reason to remain after their goal had been destroyed. They wished to report the loss of the Knowing to the High Council and saw no value in the world after it had been so "desecrated" and "soiled". They did not even bother to glass the planet.
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