Dungeon Mastery: Megadungeons

Mega means million, so conceivably it is a million dungeons. But the prospect of creating a million dungeon encounter areas is insane. you could spend a lifetime creating that dungeon. If on the otherhand megadungeon described the contents, then a million collective hit dice worth of monsters is what you are looking for.

Megadungeon: The Pit of Gnashing 

Overrun with Rats (undead rats), the Pit of Gnashing which is a significant cavern beneath the Valley of White stone, has been experiencing quakes. The rat skeletons are climbing up out of the pit and have attacked livestock and peasant alike. After battling a thousand rats that attack the village, the PCs learn they can make their way down into the pit and clear the megadungeon once and for all.

This means you can enjoy a few encounter points in the dungeon itself without putting a dent in the wildlife population.