Socrates

Socrates is the most hostile planet humans, or possibly any other race, have ever encountered. During it's "summer" the temperature sits at minus one hundred degrees celcius. During the winter equivelent, the temperature drops to minus two hundred. The surface pressure is strong enough to crush steel and Socrates is to far away from it's closest star to get any light. The atmosphere is very thin and there is a subsequent lack of oxygen or any other breathable gas. The only reason humans have established there is because of the huge amount of Slag on the planet.