Haven Base, more informally known as the Hole in the Ground, was the name given by the Rebels to the base they established in a cavern on Arbra. The base was powered by the cave's power rods. It was the main base that replaced Echo Base, after a brief layover at Golrath Station.
The cave that eventually housed Haven Base was discovered during a Rebel reconnaissance mission led by Princess Leia Organa. The remote planet seemed a good choice for the location of the new Alliance base after the evacuation of Echo Base on Hoth and the short-lived outpost on Golrath.
The Rebels soon found the planet's native hoojibs draining the energy from their equipment. The hoojibs' "spokesmind", Plif communicated with Leia telepathically to explain they had been driven from the cave that supplied them with the energy they needed for sustenance by a creature called the slivilith. After the Rebels helped the hoojibs defeat the slivilith, they discovered the immense crystalline formations known as power rods inside the cave. The crystals captured the planet's geothermal energy and technicians realized they could tap into that energy to power a new base. Their new allies, the hoojibs, agreed to let the Rebels occupy their home.
The main underground cavern and its many offshoots proved vast enough to house nearly eight thousand Rebels, their equipment, vehicles and facilities. A camouflaged, blast-shielded door sealed off the entrance grotto, where engineers built hangar facilities for Rebel ships. Additional tunnels were built to provide alternate escape routes for personnel and transports. Other facilities included briefing rooms, soldiers' and officers' quarters, laboratories, a mess hall and -- in a particularly immense cavern -- the Great Hall, used for larger gatherings and ceremonies. All these areas were organized as numbered modules.
The surrounding wilderness was regularly patrolled by camouflaged T-47 airspeeders, while Rebel sentries stood guard outside the grotto entrance. The Rebel fleet, originally maintaining orbit around Arbra, was eventually hidden inside the Arbran sun's chromosphere using a number of Kerts-Bhrg field generators.
Haven Base was maintained by the Alliance until the Battle of Endor.