Gather all "Letters of the Lost".

There's a total of 20 Letters of the Lost scattered about the islands. The Letters normal locations are in bunkers or near old artillery sites.

As you play through the game and unlock new Radio Towers, you'll notice a Letter icon on you map. These are the Letters of the Lost. You can also buy Treasure Maps from stores to help you unlock their locations. Now simply locate and grab all 20 Letters from the dead bodies for the achievement to unlock.