system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] How exactly did Gandalf and the Balrog go from the depths of Moria to fighting on a mountaintop?
gpt
A little bit more detail: 

Gandalf says at some point that nobody really knows how deep the chasm below the Bridge of Khazad-Dum is, as nobody has delved that deep. When he and the Balrog fall, they fall into a lake, and the Balrog's flames are extinguished. Gandalf mentions that in this area there are "nameless things" that gnaw at the foundation of the Earth. 

The passage describes it in a way that makes it seem like Gandalf and the Balrog are both eager to get out of there, and away from those "things", as fast as possible. Gandalf describes following the Balrog around desperately, as only the Balrog knew the way out of those areas; he alone would have been hopelessly lost. So he tails the Balrog for an unspecified amount of time until they finally reach the Endless Stair, made by the Dwarves, which lead up all the way to the top of Zirakzigil, and they fight/run to the top where they eventually do battle. 

It's unclear exactly where the Endless Stair ends. It's possible the Dwarves just stopped digging, and so below the Endless Stair is just uncharted tunnels, or the Dwarves ran into some of these "nameless things" and stopped for that reason. In any case, Gandalf and the Balrog first have to navigate dark, uncharted tunnels for a while before reaching the Endless Stair.