Cancer

Cancer's atmosphere is so toxic, only completely robotic members of The Collective with no biological components, and special reinforcement forged at Origin, can survive for more than a few hours with the planet's atmosphere. The thick clouds of acid dust hide a rich vein of ore that rules through the planet's now still core.

It has been rumored that The Source itself once visited Cancer before settling on the planet Origin, though The Source maintains that it has never left Origin, except once. People on Morpheus still swear by the story though, saying that it was so long ago The Source itself can't even remember.