Ain't Lost No More

You need to be exalted with [faction=1272] and become best friends with all the Tillers NPCs ( Friends on the Farm ) to begin doing this achievement.
Kill an [npc=59639] and loot [item=86404]. Return the quest item to Fish Felreed on the bank of the river you just killed the fish and pick up the quest Just A Folk Story. After speaking to all the citizens of Halfhill, return to Fish Felreed and pick up Old Man Thistle's Treasure. The entrance to the cave is at 37.4.24.2, behind a few bushes and is sealed with rocks which you can click to destroy. Be careful of the spiders inside the cave, they give you a nasty debuff and do a lot of damage. At the very end of the cave is the treasure, which contains one of every colour rare JC gem and a Primal Diamond. Completing the quest will lead you to The Heartland Legacy.
When you are exiting the cave it will be sealed with rocks again, this time you can't remove them though. You can either use your hearthstone or any other teleport you have, or die from the spiders and res outside the cave.
The NPC to return your final quest and receive your achievement is in a small cave in Halfhill. Have fun!

When I got to "Buddy" rep with Fish Fellreed she then had an extra chat bubble saying "Tell me the story of Old Man Thistle" which gives you some back story to it hinting at finding the map so I started reading up on this:

From a link I saw on the US forums, the Old Map is dropped by Enormous Cattail Grouper (Source on Battle.net).

It starts the quest of the same name The Old Map but as per Resania's comment below, you need Exalted with The Tillers and all the factions related to the tillers for it to drop.

You can get into the cave already however. The name is Cavern of Endless Echoes which is located at 37,23 - go to the Western edge of Cattail Lake and then NW up the hill a bit). It is blocked up but you can get in by ghost ressing (there are a few Mogu around who will "assist" you here). There is a L90 mob (Echoweb Toxiclaw]) guarding the chest but you cannot loot the treasure without the quest item - it just says "The ancient chest is covered in dust...".

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As an edit, just wanted to add it's not just 1 spider in there, there are several Echoweb Spinner on the way (had to kill 6 specifically I couldn't avoid aggroing) that spawn low hp Echoweb Hatchling as well as patrols of packs of 3 Echoweb Cinderfang which can be quite nasty especially if fighting a Spinner - if you are not paying attention their attack Burning Blood stacks up quickly - fought 6 packs of these on the way in and out and died twice thanks to them and the spinners working together - so be careful!

Also as others have commented, the cave blocks up again after you have handed in the quest and you have to hearth or die to get out.

You also get the quest The Heartland Legacy from the chest which gives you another gold reward.

Confirmed you need to be both exalted with the Tillers and best friends with all of the Tiller NPCs to start this achievement. Tried at exalted Tillers only, the Enormous Cattail Grouper didn't drop the map. Tried again when I was exalted with both Tillers and best friends with Fish Felreed, map still didn't drop. Tried finally when I was best friends with all of them, map dropped.