Sierra

The Sierra System was a star system located in the Beta Quadrant, and was presumably located near the Kaleb System.

This system is home to Starbase 39-Sierra, the closest Federation outpost to the edge of the Romulan Neutral Zone, and the headquarters for Starfleet's Romulan Task Force. The Starbase was the base for rescue and relief efforts after the destruction of Romulus, and Admiral T'nae keeps a close eye on the many starships traveling through this sector.

The planet that the starbase orbits was heavily mined in the 22nd century buy the Romulans. It is believed that the miners hit hidden pockets of deuterium, causing an explosion that split the world in two. (Star Trek: Online)