On this page of the Far Cry 6 guide, you will find a detailed description of The Truest Yaran treasure hunt in the Valle De Oro region.
District: Cielo Gardens.
Start the hunt by reading the note on the wall. You must enter the building - you can use the hole in the ground. You can jump into it or slide down the rope.
After exploring the area, you have to go underwater. Remember that your character has a very limited lungs capacity - you will be able to stay underwater for a short period. Focus, you don't want to die. Any death will take you to the very beginning of the mission.
Underwater you must find two hatches. Hit each one with a machete to close them - all the water will disappear.
Go upstairs and activate the large lever to open the passage further up. You will end up in a room with numerous posters. You can burn them all, but the most important thing is the crate lying on the stairs. Open it to end the mission.