Find Dungeons Easily
Always make sure you have weapons and plenty of torches when searching for dungeons as you may encounter a monster. Look for an area that you think may have dungeons and stake it out with torches so it is well-lit when night falls. Press 'Esc' to open up the menu and select 'Options', change the difficulty from 'Peaceful' (or whatever difficulty you are on) to 'Hard' and then close the menu. When you now press F3 ID numbers will appear on top of every animal or monster in range. Look towards the ground and search for an unusually large cluster of ID numbers underground. This is more than likely a group of monsters and will either lead you to a cave or a dungeon.
Added by: Sanzano Mar 12th 2012, ID#16982