Mercury is an oddity in the solar system in that one face constantly locked toward the sun. This has produced a number of static environments on the surface. Sunward there exists a desolate, scorching desert, while on the dark side there exists a bitter frozen waste locked in a perpetual winter. The only habitable surface of Mercury is the twilight zone. This region, trapped between the two extremes, is habitable and supports a variety of life forms. The band of habitation spans approximately 80 miles in a ring around the planet. Its main feature is a river valley that flows from pole to pole. It is in a section of the river valley where the Vulkans have their last enclave, Sheera Kress. Several British ventures have been established on Mercury. The largest is a metals pumping operation. There exists on the hot side of Mercury a Tholion settlement of which little is known.