Treasure hunting and Dungeon crawling consists of either:
Locating a dungeon or cave complex and fighting/exploring/mining your way through. We have a dungeon plugin that creates auto-resetting dungeons containing automatically replenished chest loot. These also contain secret doors, and a very maze like. These can be quite time consuming and difficult to beat - so bring weapons and friends! There is a dungeon under the dark fortress and possibly hidden on planet Acadar. Grimsgate may also contain one somewhere.
Treasure hunting - frontier planets contain thousands of automatically created ruins which contain treasures such as chests or gold blocks or rare construction materials. Getting the treasures back to the main world on the other hand, may be difficult, but you could sell them back to server, give/sell them to an npc or possibly use a tardis and an enderchest to transport them back to the main world…
