Mars is the fourth planet in the solar system, and linked to the Shade Realm of Forces.
Mars was once inhabited, and thus has a Penumbra, where one can easily see the ruins of canals running along old ley lines. Even in the Penumbra, however, the air is thin and severe dust storms are a hazard.
The Hermetic Chantry of Doissetep was translated to Mars, and the chantry itself resides halfway into the Shade Realm, though the surrounding village is firmly in the Penumbra. The process of moving the Chantry wiped out most of the ruins of the largest Martian city; the Sons of Ether and Dreamspeakers established a separate base, Marsbase One (or "Bradbury") to investigate what ruins remain.
A small defensive installation is located on Mars' smaller moon, Phobos, to prevent Technocracy infiltration.