Balrog Breaking and Entering

[Inziladun]

Originally Posted by Nerwen

I find it more likely that it entered through the tunnels beneath Moria– you know, the ones "gnawed by nameless things" (see The White Rider).
According to Gandalf, the Balrog was familiar with these. We're not told if there was a way directly into these deep tunnels from outside at the time of the War of the Ring, but I don't see why there couldn't have been at some point.

My only problem with this is you'd think that the entire area, not just Moria proper, would have been crawling with Dwarves. There should have been constant travel to and from the West-gate through the area later to be known as Eregion.
I would say the Balrog was lucky to have escaped their vigilance, but it was probably the Dwarves who were luckier.
This actually makes me wonder why the Balrog picked a location where it was likely to be discovered sooner or later. The Misty Mountains were a pretty large range, and you'd think there would have been hundreds of caves it could have explored. Maybe it had once known the Watcher and wanted to catch up on old times.