A Federation mining colony of some five hundred people that suffered a devastating underground explosion in 2364, the emergency during which Q tempted Riker with the powers of the Continuum.
Quadra Sigma III is the site of a Federation mining colony in the Sigma III system. It appears in the first-season Next Generation episode Hide and Q.
In 2364 an explosion of methane-like gas tears through the colony's workings, trapping and injuring many of its 504 inhabitants. The Enterprise-D races to help, but on the way Q halts the ship and pulls the bridge officers into a game on a barren world, at the end of which he grants Riker the power of the Q.
When the ship finally reaches the colony, an away team digs survivors out of the rubble. Among the dead is a young girl. Riker, who has promised Picard not to use his new abilities, stops himself from restoring her to life, and the anguish of that choice pushes him to test the powers he has been given.