Derna was a moon orbiting the planet Bajor.
During the years of Cardassian occupation of Bajor members of the Bajoran Resistance often hid in small vessels and pressure domes on the moon.
Dominion WarEditDuring the Dominion War, the Romulan Star Navy established a hospital on the moon.
When evidence surfaced that they had armed it with ground-based plasma torpedo launchers, the Bajoran government demanded they be removed, fearing that the Romulans would try to claim the moon as their own one day. The Romulan government refused, and a standoff between Bajoran Militia forces and Star Navy ensued. Starfleet Admiral William Ross threatened to remove the weapons if the Romulans wouldn't, and the Romulans relented.
After the war, the hospital was converted into the headquarters of the reestablished Bajoran Space Guard while construction of an orbital facility began in March of 2377. After the station was completed it remained a training facility for the Guard's medical personnel. ("Gatekeeper")