Have I ever said that I LOVE dungeons? Yes, dungeons. Those strange twisting collection of passageways through the ground, or sometimes even through more sinister realms. Yes, I know that they are (perhaps) unrealistic. I know that certain gamers look down their noses at them.
But, come on. If our suspension of disbelief can accomodate dragons, elves, and dwarves, CERTAINLY it can accomodate huge labyrinthine complexes of caves and passageways carved through the earth. Well, mine can.
Then there are those gamers who can get past the "idea" of the dungeon, but then get hung up on the unrealistic nature of most dungeons that have been published, or even that haven't been. There are basically two schools of thought that have attempted to answer this problem. Of course, there is a third way: Ignore the problem, because it isn't one!
The first school says, "Well, if you think that dungeons are unrealistic, make them more realistic!" A DM would do this by, for instance, including latrine facilities for the creatures that dwell within, ensure that the sources of food can support populations of a given size, etc, etc. The DM would attempt to determine why creatures are where they are located in the dungeon and ensure, to the best of his ability, that his reasoning is plausible.
The second school says that there is a reason that the dungeon does not function in a way that makes ecological sense. This school is best described by Philotomy here. He sees the dungeon as a 'mythic underworld'. I have to say I had never thought of this idea, and I like it a lot. It allows a lot more freedom for the DM, but goes one further than the Third way mentioned above by actually giving a reason for the dungeon to be as it is.
Regardless of your school of thought, I think that there ample real-world examples of dungeons, and if they can exist in the real world, then they can CERTAINLY exist in a world populated by dwarves, and all manner of other subterranean creatures. And, going further, of one assumes several millenia of history, then these races had generations and generations to craft, expand, and carve out underground locations ripe for "present day" adventure.
I guess my point in all of this is that dungeons are awesome.
And in case you didn't guess it, there is only one thing better than a dungeon, and that is a map of a dungeon!