: Did they necessarily carve the whole thing out? Or did they just expand upon a natural cavern system within the mountain?
: Also, I expect that Moria expanded along the lines of the dwarven population increase - and therefore you have several thousand dwarves undertaking an ongoing labour over the centuries....
: Dave

I think that the abyss Gandalf fell into with the Balrog, as well as the tunnel system connected to it, were definitely not dug by the dwarves. That suggests that perhaps much of Moria was composed of natural cave, merely fashioned by them.