From the website:
Once, no man on Earth thought that life on Mercury was possible since the planet is locked with one side perpetually facing the Sun. Scorching heat on one side and eternal darkness on the other seemed to guarantee that Mercury must be uninhabitable. But when the inhabitants of Earth first started to voyage through the Ether, some daring explorers also turned towards Mercury—and discovered how wrong the previous assumptions about the planet had been.
Because even if the one side of Mercury is dominated by vast hot deserts with lakes filled with molten metals and the other side is a seemingly lifeless waste with its giant glaciers of ice and ammonia, there is still a small strip of land between these two extremes that thrives with life. This zone where the cold and hot sides meet always stays in the same twilight and at the same temperature—this is the realm of the mighty World River, which spans the whole planet and is a lifeline for the primeval fauna and flora on both river banks.
While the few human settlements and bases were all established along the World River, the main focus of the great powers and trading companies remains on the hostile areas of Mercury—because that is where the true riches of the innermost planet of the Solar System lie. Lakes of molten metal await their exploitation on the Hot Side, if only there were a way to mine them. Frozen gases on the Dark Side also promise great profits, despite all risks to life and limb. It is no wonder then that it’s not just scientists that come to Mercury, but also salesmen and prospectors, just as some inventors try to construct machines that would function in environments more life-threatening and hostile than anything known on Earth.
This supplement describes everything you need to know to have thrilling adventures on Mercury. Detailed descriptions of the three very different regions, information on inhabitants and visitors of Mercury, interesting locations, or alien (and not always animal) life—this sourcebook offers you a comprehensive survey of the planet.
To help you dive into this exciting world more easily, this book also includes an adventure concerning a race along the World River which lets you and your players explore large parts of Mercury during its course.
The alien and fascinating world of Mercury awaits you!