Calth is an Imperial Civilised World in the Veridian System and it is a part of the Realm of Ultramar that is ruled by the Ultramarines Space Marine Chapter and lies in the Ultima Segmentum. As part of Ultramar, Calth provides recruits for the Ultramarines Chapter. The Aspirants compete in a series of contests between hopefuls to determine who is worthy of joining the Space Marines. Calth's inhabitants live in subterranean hive cities where the deadly ultraviolet light of Calth's blazing blue sun cannot reach them due to the planet's lack of an atmosphere. The caverns of Calth are constructed on such a huge scale, and with such grandeur, that they are as light and airy as any city of Macragge. The world above is an arid wasteland devoid of air and covered in the ruins of shattered cities.