The first ships landed on Mercury in 2021. While orbital probes had mapped the arid world decades earlier, it took a while to develop space suits that could endure the extreme heat of the surface. The first expeditions were scientific, as the tiny rock was an excellent place from which to study solar phenomenon. However, the discovery of the Shadow Caves in 2026 gave Mercury a new purpose.

Millions of years of turning from the blazing heat of the day to the endless chill of night has pitted and cracked the ground of Mercury. Below the surface, the ceaseless expansion and contraction has created a series of caves pervaded by a little water, oxygen, and life. Called the Shadow Caves, they seem to be an endless black expanse of thin, hot air and twisted, flesh-ripping knives of stone.

Mercury is rich in radioactive isotopes, especially uranium, with enough fissionables to power the System for as long as anyone can imagine. The extreme harshness of the planet made mining seem nearly impossible, until the Earth League decided that Mercury would be the perfect place to deposit the worst criminals of the System. While the traditional Earth League approach to crime is reform and recompense, there were always some criminals too hardened, twisted, or brutal to ever be returned to society, as well as some crimes that could never be compensated for. So the Mercury Mining “colony” became the last stop for the vilest of the System: pirates who butchered the helpless crews of the ships they caught; those who stole life-support gear from Lunar and Belt habitats and left the inhabitants to die; terrorists who targeted civilians; traitors to the Solar Patrol; and willing allies of the enemies of mankind. There are not many cruel and twisted enough to be shunted here – the population is rarely over 500 – the mere threat of it keeps at least some on the straight and narrow. The average life expectancy of a prisoner on Mercury is five years.