During the very first sequence you can unlock this achievement. After you rescue the Merchant as part of the story, you'll be told to go to his ship. Don't. Instead, go to a Cadaver found on the map. This gives you a treasure map, and luckily the treasure is in the same area you're in. Once you've grabbed the map, head south towards a cave. Inside the cave is an X, which will prompt you to dig using the B button. Once you crack open the chest the achievement should pop!