When killing monsters in the dungeons they will sometimes drop treasure maps. Those maps mark an entry to a cave where you can fight against monsters which are much more powerful than the dievils and t-rexes which you find in normal dungeons. When killing all monsters of a treasure cave you will also get some money (the treasure), but there is a time limit of 10 minutes. It is recommended to only dig for those caves when you are powerful enough or have some friends helping you to kill the monsters. The new monsters are stronger, but also give more experience points!