The Frontier was the place to which Althena exiled the Vile Tribe over five hundred years before the events of Lunar Silver Star Story Complete. It is a barren wasteland with craters and little water. It is sometimes known as the "desert of death". The Grindery was built and activated there.
It is said that it is the edge of Lunar. The Blue Star is not visible from the Frontier, indicating that the Frontier is actually the side of the moon that does not face the Earth.