Mars

Mars is a once Earth-like planet which atmosphere was stripped away from it due to its lack of a strong magnetic field.

Colonization
In 2214 A.C., the Earth's population had grown to more than 15 billions, rendering the planet's resources insufficient. Seeing this, the UNSP launched a mission in order to start a colony on the cave known as "Dena", near Arsia Mons, that colony was later called Aegis City.