Old Man Thistle's Treasure

Quest progression:

1. 
   
2. The Old Map (from [item=86404] looted from [npc=59639] if you are exalted with [faction=1272] and are best friends with all the farmers Friends on the Farm)
3. 
   
4. Just A Folk Story
5. 
   
6. Old Man Thistle's Treasure (rewards [item=86428] which contains one of each rare gem)
7. 
   
8. The Heartland Legacy (achieves Ain't Lost No More)
9. 
   

Once you are in the cave there are three possible bugs that can happen:
1. Exit it blocked. Solution: Have to hearth out when quest is completed.
2. Fish disappears. Solution: Relog.
3. Grey ? over the treasure chest after you kill the big spider. Solution: Fish has to be present when you kill the spider. If she is not, follow step 2, wait for the spider to respawn and kill it again.
All these bugs happened to me so that's how I know >.<

Listening to Fish Fellreed's story, she talks about a lost map, [item=86404] is this map, which is found in Valley of the Four Winds, and which starts a quest. The Map quest will eventually lead to getting this quest and going into the blocked off cave in the North of the zone.