From Halopedia, the Halo wiki
Lemuria was one of the six continents on the planet Arcadia, a human colony in the Procyon system governed by the Unified Earth Government. It was abandoned after the Battle of Arcadia in 2531 and destroyed by the Covenant in the Fall of Arcadia in 2549.
There was six existing provincial regions on the continent.
The continent was likely named in reference to Lemuria, a mythical lost continent on Earth.