yeah I got in there today as well, pretty interesting cave but I was disappointed when I couldn't get the treasure.
You have to be exalted with The Tillers and must acquire the Friends on The Farm achieve where you reach best friend status with all the friends. Then, you kill the large Cattail Grouper in the lake where fish felreed stands on the docks. It will drop a map, then you have to do one or 2 small quests then you are allowed to open the cave; go all the way to the end, kill the large spider in front of the chest and complete the quest. The chest will not open, but upon finishing the quest you will receive a small chest in your bag to open. Mine contained 8 crystals and 100 gold. :) Once you turn in that quest you get the "Ain't Lost No More" achievement.