system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] How did the Balrog get beneath Moria?

Khazad-dûm was a Dwarven kingdom established well before the 1st Age. How did the Balrog delve so deep under Moria after the War of Wrath, and without any one noticing, even though there was already a Dwarven Kingdom there.
gpt
Morgoth delved deep into the earth to create his fortress, and it's therefore quite possible he had tunnels and passageways all under Middle-Earth. The Balrog, fleeing the destruction of Morgoth's forces through these passageways could quite possibly find it's way to Moria, which is not really that far from where the fortress was said to be.