Corin (Kilian IX) is a mesoplanet in the Kilian system, United Empire of Earth (UEE), owned by the UEE Marines. Located far outside the system's green band, surface conditions on the planet are very dangerous for anyone without protective gear. The Marines maintain an extensive underground base here, used as their headquarters and primary training ground. The exact nature of their training is classified, but many believe that the planet's inhospitable environment factors heavily into it. A full Marine battle group patrols the area around Corin at all times to prevent unauthorized access.

Corin was discovered in SEY 2463 in the initial survey of the Kilian system. As Human territory expanded from Kilian to Ellis, from Ellis to Magnus, then from Magnus to Terra, Kilian's connection to these systems caused its strategic value to rise. Multiple branches of the military were among the first colonizers of Kilian, starting with a forward operating base from the UEE Navy on MacArthur (Kilian V). In 2560, the newly independent UEE Marines chose Corin, an inhospitable, frozen dwarf planet, to use as a base for extreme condition training operations.

Corin's position beyond Kilian's frost line made it impossible to terraform. The Marines instead established a sealed colony as their initial base and soon expanded underground with factories, water processing plants, and waste recycling facilities staffed and run by Marines and high-level clearance contractors. Additionally, budget reports from 2611 revealed that arcologies for raising livestock and growing produce were added to Corin's surface. In 2633, construction appeared to be complete and all imports to the planet ceased.

Today, Corin remains entirely within the purview of the Marines. The planet is surrounded by a no-fly zone and is patrolled at all times. Ships that have not been cleared to pass through Corin space are subject to shoot-to-kill orders. Non-military ships that have been cleared to land by High Command are only allowed to land on Corin's surface with the stipulation that they do not enter the underground base. Because the base is self-sufficient, this happens very rarely.

All Marines selected for service from the Army or Navy go through six months of rigorous training on Corin. While the majority of a recruit's training takes place underground, wargames and other training exercises are conducted on Corin's harsh surface to prepare each recruit for the rigors of fighting in potentially deadly environments.