Consider what happens in book2 chapter 4:
The next day, the group travels to the west and Gandalf opens a magic door. These doors close just in time for the group to escape an attack by one of the vicious creatures that live in the nearby lake. Now safe from tentacled beasts and howling wolves, the travelers are at the mercy of the miles and miles of caves that stretch before them. They are in the cave system for two nights and Gandalf is the only one who can navigate and lead the group. The caves of Moria spark an ancient tension in the group because the dwarves once mined Moria for valuable metals but they have lost the territory. The tombstone of Balin the dwarf explains the story of one unsuccessful dwarf who tried to reclaim Moria.