The Aprox. Loc for the underwater Atlantis Library is at 40k 60k in Messo.
Guarded by Triton Guards ...
We attempted this one time so far and failed with 1.5g Badly. Once you open the Correct Chest The Atlantis Guardian pops out. Not sure how or what, but some of our actions made her split, causing Atlantis Soldier guys come 2-3 of em, and Really tough. Also got spams from some of our actions Causes her to Heal her self. Have yet to figure out what we did or how to prevent.
Just a little info for your heads up. Id say take 3g + to be on the safe side. If you fail, and go back, The chests took over 24hours to repop. So make your first try count :)