Mayan Stela #9: Matanzas (343, 642)
Keep en route to the mission, and you'll pass by the Matanzas plantation. Dock here and head southeast from the pier. You'll have to sneak through a restricted area, but there's a path on the eastern edge of the plantation that takes you straight here.
Mayan Stela #10: Tortuga (875, 379)
After completing the The Forts mission, you'll be just a bit northwest of Tortuga. This is probably the most heavily guarded area in the game, but it is possible to sneak in. That being said, you may want to upgrade your ship - and we've got a guide for that! - and fight your way through. In either case, you'll find the stela in the northern area of the plantation.
Mayan Stela #11: Pinos Isle (334, 476)
The next mission is located down south, but we're going to take a detour around the landmass first. Head west to Pinos Isle, which is restricted and guarded…by two measly frigates. Taking them down to access the isle shouldn't be a challenge for you by now. Follow the path from the shore to find the first stela slightly southwest from the massive step pyramid.
Mayan Stela #12: Pinos Isle (342, 478)
There's a second stela on this island. You'll find it at the northeastern edge of the isle.
Mayan Stela #13: New Bone (431, 116)
Continue on this wraparound detour until you near Misteriosa. Southeast of here is an island plantation called New Bone. Just like Tortuga, it is possible, though very difficult, to sneak in here. Be stealthy in the approach or strong on the attack, and you'll reach the shore. The stela is just outside the restricted area in the northwestern section of the island.
Stela #14: Isla Providencia (502, 44)
You'll end up stranded here during the mission Marooned. As you chase Vane, make a left when you reach the ruins to spot this stela.
Stela #15: Isla Providencia (502, 44)
There's another stela here. Once you take out Vane, you can return to the island to explore the second set of ruins - just be aware that they're now a restricted area. You'll find the stela in the back of the area.
Stela #16: Long Bay (525, 253)
You won't be able to access Long Bay until the mission The Observatory. When you reach the last jungle in need of clearing, look in the southern area by the large black structures to find the game's final stela.
The Mayan Outfit
With all 16 Mayan Stelae puzzles solved and stones in pocket, return to Tulum. Head to the indicated spot on the mini-map (it's underneath the step pyramid) to place the stones. Grab the Mayan Outfit inside, and enjoy your new immunity to all ammunition!