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