Unsolved Mystery is the name of a fan fiction that spun off of the TV series Sliders. In it, several individuals who had once been kidnapped by the Kromaggs (the series' villains) band together to defeat an invasion on the world where they currently live. After that, they also spent much time visiting other worlds and helping those in need.
Unsolved Mystery was often abbreviated by the writers into USM. The group of sliders in the story referred to themselves as "Rifters," and that name became an unofficial title for the series as well.
The fanfic was told in a round-robin style, and lasted from 1996 to 2000.