Part of the Second Age of Middle-earth Exploratory Series
When the War of Wrath ended, the great subcontinent of Beleriand in the extreme-west of Middle-earth was almost entirely destroyed and sank beneath the ocean. This caused problems not only for Elves and Men but for our bearded friends, the Dwarves, as well. They had to great cities in the Ered Luin (Blue Mountains), Belegost and Nogrod. We can see these Dwarven cities in the map of Beleriand included with The Silmarillion. (Don’t forget: the Middle-earth we know and love from the Third Age adventures of Bilbo, Frodo, and their companions lies to the East of this mountain-range and is thus not pictured here.)
After the end of the First Age the power and wealth of Khazad-dûm was much increased; for it was enriched by many people and much lore and craft when the ancient cities of Nogrod and Belegost in the Blue Mountains were ruined at the breaking of Thangorodrim. The power of Moria endured throughout the Dark years and the dominion of Sauron, for though Eregion was destroyed and the gates of Moria were shut, the halls of Khazad-dûm were too deep and strong and filled with a people too numerous and valiant for Sauron to conquer from without. Thus its wealth remained long unravished, though its people began to dwindle.¹
We should not understate the significance of the migration of the dwarves to Khazad-dûm. It was not a short journey. Using Karen Wynn Fostad’s invaluable Atlas of Middle-earth and a cord from a pair of headphones with which to measure (super-scientific, right?), I estimate that the journey from the Blue Mountains –> Grey Havens (to pick up the Great East Road) –> Rivendell –> Moria is between 970-1000 miles. I kept my measuring-device in a more-or-less straight line at each stage of the journey, so I’d put it closer to 1,000 miles to take into account the bends in the road, etc.
Update: I’d like to thank Adrian Ciobanu (via Facebook) for reminding me that, of course, Khazad-dûm was the only known source for mithril. This played no small part in its success during its peak. But of course, we cannot know for certain whether the mithril would have been found or worked as successfully, had not the number of skilled craftsmen increased so dramatically from SA40 onward.
It is perhaps worth a small sidebar here. Tolkien once said of the Dwarves:
I do think of the ‘Dwarves’ like Jews: at once native and alien in their habitations, speaking the languages of the country, but with an accent due to their own private tongue. . . . . [Letters, no. 176]²
The wandering Dwarves are said to have arrived in Khazad-dûm c.40SA [Appendix B]. It is perhaps notable that that it took 40 years for both the Dwarves escaping the ruination of the Blue Mountains and the Jews escaping the persecution of Pharoah to reach their new homes (Moria and The Promised Land, respectively). Granted the Dwarves are not said to have ‘wandered’ for the entirety of that 40 years. Indeed, there was a (more-or-less) straight road, and they knew precisely where they were going. Still, it is notable.
The above map, taken from the Atlas of Middle-earth, highlights some of the migrations taking place following the War of Wrath. Of course, the Dwarves were not the only ones to relocate to Moria! As we can see, at least one Balrog escaped from the destruction of Thangorodrim and it made its way to Khazad-dûm where it lay hidden for more than 5,000 years. As Fonstad reminds us:
The only Balrog mentioned in later times was the one that fled to Moria and was found in the early Third Age by the deep-delving Dwarves. There it stayed until it was slain by Gandalf.³
According to Appendix A & B, it was around TA1979-80⁴ that the Balrog was revealed in Moria. Durin (VI) was slain (hence, Durin’s Bane) as was his son, Náin.
…and then the glory of Moria passed, and its people were destroyed or fled far away. ¹
- The Return of the King , Appendix A, III, ‘Durin’s Folk’
- The Letters of J.R.R. Tolkien 
- The Atlas of Middle-earth (Revised Edition), Fonstad, 
- The Return of the King , Appendix B, ‘The Tale of Years’