For the life of me, I don’t know what there should be so many dungeons lying around. Do you?
- Giant worms (wyrms?) dug tunnels under the ground long ago. Various peoples occupy the larger ones, building towns. The smaller ones became hideouts for theives and hermits.
- They were dug as meteor shelters in days long past. When the planet and moons are in a certain alignment, meteors tend to fall. When that time comes, the people head below (where the monsters await). Thieves take their chances up top. That doesn’t always work out so well.
- If you are in a post-apocalyptic mood, the tunnels were once transport tubes for trains. Check out nycsubway.org for maps and pictures. Now put the monsters in. You don’t even need to tip off that the party is exploring underground Manhattan. Just remove all the tracks.
- They are lava tubes from that gigantic volcano over there. You know, the ‘dormant’ one?
- The planet is made of cheese.
Feel free to post your own ideas below.