Vault Raider Achievement

The secret door is hidden in a tunnel at Tulum (70, 405). In order to unlock it, you will need to collect all 16 Mayan Stelae which are buried around the map. Fortunately, they are marked and it is mostly a matter of traveling to the locations for each one. When you reach a location that has a Mayan Stelae, there will be a totem pole-like structure to climb, and at the top you will need to press to interact with it. This will activate a mini-game where you need to align three white shapes with nearby terrain features and is mostly a matter of rotating until it clicks into place. When that is done correctly, you will reveal the exact location of the buried Mayan Stelae. Go to the indicated spot and press to dig it up.

The 15 Stelae not obtained in story mode are located as indicated below:

• Cape Bonavista (179, 593)
• Cat Island (742, 695)
• Great Inagua (845, 468)
• Isla Providencia (502, 44) - 2 Stelae
• Long Bay (525, 253)
• Matanzas (333, 650)
• Misteriosa (307, 195) - 2 Stelae
• New Bone (442, 118)
• Santanillas (217, 250) - 2 Stelae
• Tortuga (882, 370)
• Tulum (70, 405)

After you have all 16 you can enter the tunnels underneath the only viewpoint at Tulum and use the Mayan Stelae to open the secret door. Once you have opened it once, it will never open again.

Here is a video showing the locations of each Mayan Stelae and the secret door: