The Sigma III system was a formerly inhabited star system close to Starbase G-6. In the mid-24th century, the United Federation of Planets had a mining colony on Quadra Sigma III, a planet in this system.
In 2364, the starship USS Enterprise-D received a distress call from the colony following a catastrophic mining accident. The Enterprise-D rescued nine survivors from the ill-fated colony. (TNG: "Hide and Q")