WHERE IT STARTS:
There is the abandoned MCKAY GLOBAL DRILL SITE D in the south western corner of LA JOYA.
Interact with the orange box at the cave entrance to start.
THE NOTE SAYS:
"The tunnel rescue team has been declared lost and this drill site is condemned, effective immediately. Bring your keys to the on-site office for collection.
All shrine artifacts recovered from this loation (Drill Site D, findings: Mimo Abosi) are the property of McKay Global. Every employee must sign an NDA. What happened in the tunnel, stays in the tunnel!
J. Ricciardi, Acting Site Supervisor."
WHAT TO DO:
After reading the note, go up to the office container on top of the drill site.
Smash the wooden door a couple of times to get in and take the key.
Now go down to the closed up drill site with the Treasure Hunt orange box.
Destroy the orange generator to cut the power, so you don't have to come back later.
Then, melee smash the lock on the gate and go in.
At the first crossing go left. There is a Hidden Histories Collectible called "Devine Intervention #4" on one of the wire rolls. You can also pick up another key on the floor.
Then go back to the crossing and down the stairs. The flooded area would be under electricity. If you have not blown up the orange generator, you have to do so.
Get in and dive through to the other side.
On the other side, get out of the water and open the closed door with your keycard. There is a switch where you can turn on the ventilation system.
After you've done that, grapple down the elevator shaft.
Finally you can get into the big room, where you can pick up the idol and the special gear in the chest.