The Criptograma Chests are a new feature to the Far Cry series that requires you to collect Criptograma Charts to unlock them. One of these chests resides in the sunken tower in Cielo Gardens. Take a look at the map below for reference. Cielo Gardens is located in the southeastern section of Noventarmas. It is a couple of hundred meters north of the Dulce Sugar Mill FND Base.
When you reach the area, locate the large tower in the center. It is easy to spot. You will need to swim over to it. There is a dock at the base of the tower that you can climb on to find the chest.
Once you have located the chest, you will need to climb the tower. Take the ladder to the right to begin your climb. You will need to use your grappling hook a few times and climb some vines as well. When you reach the top, you will see the first Criptograma Chart.
From the top of the large tower, you will be able to see a smaller tower. If you have the wingsuit unlocked, you can fly over there quickly. If not, you can still use the parachute. Climb to the top of the smaller tower to find the second Criptograma Chart.
With both of the charts discovered, you can now head back to the large tower and collect your loot from the chest.