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Ain't Lost No More
Return Old Man Thistle's lost treasure.
Ain't Lost No More is a 10-point achievement earned by completing the quest chain that culminates in returning Old Man Thistle's lost treasure from the Cavern of Endless Echoes in Valley of the Four Winds.
Reach Required Reputation
You must be Exalted with The Tillers and have Best Friend status with every individual Tillers NPC (the Friends on the Farm) before the quest chain will become available. Being Exalted alone without all Best Friend statuses is not sufficient for the 
Old Map to drop.
Kill Enormous Cattail Grouper
Head to the lake next to Fish Felreed and kill the Enormous Cattail Grouper — it will have a quest icon on its nameplate — and loot the Old Map from it.
Return Map to Fish Felreed
Bring the Old Map to Fish Felreed on the riverbank where you killed the fish and pick up the quest Just A Folk Story from her.
Speak to Halfhill Citizens
After accepting Just A Folk Story, speak to all the citizens of Halfhill, then return to Fish Felreed to pick up the follow-up quest Old Man Thistle's Treasure.
Enter Cavern of Endless Echoes
Travel to the Cavern of Endless Echoes at 37, 23 on the western edge of Cattail Lake, then head northwest up the hill. The cave entrance is hidden behind bushes and sealed with rocks that you can click to destroy. Be cautious of the spiders inside, as they apply a heavy debuff and deal significant damage.
Loot the Treasure Chest
At the very end of the cave is Old Man Thistle's treasure chest, which contains one of every color of rare Jewelcrafting gem and a 
Primal Diamond, along with a quest starter for the follow-up quest chain.
Complete The Heartland Legacy
Loot the chest to receive the quest starter for The Heartland Legacy and complete it to earn the Ain't Lost No More achievement.
Tips
See the video guide at https://youtu.be/Am3uHb2CU0A for a full walkthrough of the quest chain.