Aglarond

The gorge of Helm's Deep ended in the mountainside, and there opened a series of caves that ran far back into the White Mountains. To the Rohirrim they were simply the caves of the Deep, a place to store grain and fodder and to hide the women, children and old in time of war, for a few men could hold the narrow entrances against many. The people of the Westfold spent the night of the Battle of the Hornburg within them, and it was to the caves that Éomer and Gimli were driven when the Deeping Wall fell.

The Elvish name Aglarond, the glittering cavern, was given by Gondor, whose builders first fortified the Deep, and the name was afterwards used for the fortress as well. The caves deserved it: Gimli, a Dwarf of Erebor who had seen the halls of his own people, described chamber after chamber of crystal and veined stone, columns rising from still pools, and walls that shone in torchlight with colours the Rohirrim never troubled to look at. He judged them among the wonders of Middle-earth and grieved that the Men of Rohan used them as a cellar. He promised Legolas that he would show them to him after the war, in exchange for a walk together in Fangorn.

The caves had sheltered the people of the Westfold since the days of Helm Hammerhand, who held the Hornburg through the Long Winter of T.A. 2758. During the Battle of the Hornburg on the night of March 3, 3019 the people took refuge in them, and Gimli, cut off with Éomer and Gamling when the wall was breached, fought at their mouth. After the War of the Ring Gimli brought a company of Dwarves south from Erebor and founded a new dwelling in the caves, becoming Lord of the Glittering Caves. His folk did great works there and in Rohan and Gondor, and it was they who forged the gates of mithril and steel for Minas Tirith.

The caves opened at the inner end of the gorge of Helm's Deep, behind the Deeping Wall and the Hornburg, and ran back beneath the Thrihyrne.