Structure:
%Roll: Structure types:
01-10: Ancient Ruins
11-20: Catacombs (this will be un underground ancient burialgrounds, and is man-made)
21-30: Cave
31-40: Crypt (man-made)
41-50: Dungeon
51-60: Gate
61-70: Labyrinth
71-80: Mine
81-90: Tomb
91-00: Underground volcanic tunnels