The Glittering Caves, or the Aglarond, as they were known, are the caverns at the back of Helm's Deep. They were originally worked by the Numenorians and later used as a refuge and for storage by the Rohirrim.
During the Battle of Helm's Deep, Gimli fought in these caves and discovered their great beauty. After the war he returned to them with other dwarves from Erebor, and he became Lord of the Glittering Caves. The new gates of Minas Tirith were forged there, and the dwarves continued to make many fine things for both the Gondorians and the Rohirrim.