Once Fish Fellreed gets to know you better, she will tell you the story of Old Man Thistle, a farmer who found a secret cave full of treasure. It collapsed behind him after his mushan Bobo smashed through a wall, but he drew a map to its location. Unfortunately, the location of the map, like the cave, has been forgotten.
In order to start the questline, you need to be exalted with the Tillers faction and Best Friends with all of the Halfhill Market characters. You'll know you're done when the Friends on the Farm achievement pops up.
There's no quest for this, but you should head to Cattail Lake and hunt the Enormous Cattail Grouper. Be careful, though -- he's big enough to swallow you whole. (These groupers are also the target of the Gift for Haohan daily.) If you meet the requirements, the grouper will drop an Old Map and launch you on a brief but lucrative adventure.
Filed under: Mists of Pandaria