A Death World is a planet which is too dangerous for a variety of environmental and biosphere reasons to support widespread human settlement. The types of Death Worlds are varied, ranging from planets that are covered by world-wide jungles that harbour vicious carnivorous plants and animals to barren rockscapes strewn with volcanoes and wracked by ion storms. These worlds are near-impossible to colonise by Mankind due to their environmental conditions or the nature of their biospheres. Nonetheless, many of these worlds have large human settlements, which are notable for the strength and self-reliance of their people. Many of the people of these worlds are inducted into the Imperial Guard, or recruited by the Space Marines, a fact which is often the sole reason for the continued habitation of those worlds. Some harbour rich mineral, vegetable, animal or gaseous resources that are of such value to the Imperium that a small human settlement will be maintained despite all the dangers. It has been theorized by the Imperial Magos Biologis of the Adeptus Mechanicus that many Death Worlds are the result of biological seeding by ancient advance Hive Fleets of the Tyranids that entered the Milky Way Galaxy long ago. Many of the monstrous creatures inhabiting these worlds, such as the Kraken of Fenris and the Catachan Devil of the jungle world of Catachan, have many features in common with Tyranid organisms, and may in fact be descended from them. Left behind by the Hive Fleets that seeded these worlds and then moved on, these Tyranid species lost access to the Tyranid Hive Mind and so degenerated into unintelligent yet vicious animal species.