Large dungeon beneath island

[cartejos]

I have been working on a new setting for my group. I want the main dungeoneering bits for when the more fight oriented of the group get bored and I thought it wouls be cool to have an ancient system of passages and dungeons below the island.

The entrances to the dungeon system are located in the mountain cave systems. Their are two mountain ranges on the northern and southern sides of the island.

To make it simple the island can be thought of as circular with a diameter of 170 miles. I thought that with the mountains being off the coast this would mean somewhere along the lines of 100-150 miles of dungeons that could be played out below the surface.

I would appreciate any input as I would like to make this as interesting as I can for my players.

[Toliudar]

I'm not clear on what aspects of this plan you want advice/suggestions for. What races/social structures/monsters to put down there? Adventure hooks? Because with the scale you're describing and the potential for multiple layers, you can effectively have as much dungeon as you want, and then some.

For a cave structure on the scale of what you're describing, the various versions of the Underdark, and its associated sourcebooks, might be excellent starting points for ideas.