Today's Date in the Shire
0 2 Lithe
Fun and Games
Walk to Rivendell
Malbeth the Seer
The old dwarven city beneath the Misty Mountains, 'Khazad-dûm' in the tongue of the dwarves.
The neutral translation into Sindarin, 'Hadhodrond', which is 'Dwarrowdelf', was never used much among elves, especially not the Sindar. They expressed their horror and dread of this place, even long before it was occupied by the Balrog, by the name of 'Moria', which means 'The Black Chasm'. This was a name even the valiant men such as Aragorn were reluctant to utter.
When the Fellowship entered from the west gate, they had to face two days of marching through a dark maze of tunnels, stairs, halls, chambers and chasms full of unknown dangers. Only under the guidance of a leader such as Gandalf could they pass such a trial, and before hand he needed to give everyone a good sib of the miruvor.
According to the tales, it was already Durin, legendary forefather of the dwarves, who settled in the caves above the Azalnubizar (Dimrill Dale) and the Khled-zâram (Mirro Mere). Over the course of the successive millenia, Khazad-dûm expanded in many storeys almost over the whole massif of the mountains Celebdil, Caradhras and Fanuidhol.
In this time, the west gate of Moria was built in order make the trade with eregion easier and faster. Elven influence was seen on the tree-shaped pillars in the second hall before the east gate. Few is known about how close the dwarves cam into contact with Sauron back then, but Durin III at least got one of the seven dwarven Rings of Power. In the war of the Last Alliance, the dwarves of Moria were among the few Naugrim who fought, and they at least fought against Sauron.
In the Third Age, the dwarves delved ever deeper in their search for mithril. There they unasked for came across a Balrog, who had taken refuge there after the fall of the Thangorodrim. The dwarves deserted Khaza-dûm after King Durin VI and his son Náin I were slain by the Balrog in 1981 TA, and in later years, Moria became a settlement for many of Sauron's minions. Other previously unknown creatures, the mightier ones not under the control of the Dark Lord, moved there, too, or climbed up into higher levels. One of those was the Watcher in the Water, a dangerous creature whose massive bloat of body blocked the gate-stream Sirannon to the west. Only closely could the company escape its stong tentaclous arms.
At least two attempts were undertaken by the dwarves to reconquer and reestablish Moria. The first effort, in which Thrór fell, led to a big campaign out of vengeance against the orcs. Although the orcs under Azog were utterly defeated in the Battle of Dimrill Dale, 2799 TA, the fear of another confrontation with the Balrog kept the dwarves off Khazad-dûm for some more decades, before greed forced them again. The casualties the dwarves had to suffer in the batlle were also so great, that they had to burn the fallen instead of properly burying them after dwarven manner.
Balin's attempt to regain their old city with a number of dwarves from Erebor was as well doomed to fail. Their records Gimli found in 3019 TA in the Book of Mazarbul, which told of the colonization and the final defeat against the overwhelming tides of orcs.
At the Bridge of Khazad-dûm near the eastern gate, the Company encountered the Balrog, whom Gandalf took on in a duel which was dominated by their great magical powers. Durin's Tower atop of the Zirak-Zigil was destroyed as a consequence of it. Although both combattants died, Gandalf was sent to Middle-earth again shortly after.
It appears likely that Moria was retaken by the dwarves after the War of the Ring, since they possessed considerable amounts of mithril again at the rebuilding of Minas Tirith.
References: FOTR, II, 2-5; appendices a, b; Silm., 'Of the Rings of Power...'