Sunken Crypt (Location)
NOTE: It's possible to fast travel underwater, so don't worry about having to save breath for the way back.
One of the trickier treasure locations, this one in Memphis. Scans will show the lone treasure pretty deep underground.
Enter through the front door, and follow the stairs until they submerge completely.
Dive down, and take a left past some broken pillars. Your pulse will show a long tunnel in this direction.
Follow it for a few dozen meters, until there's a drop. Swim down, then south-by-south eastish through another, lower tunnel. This one curves east into a stone wall, with a square opening.
The treasure is in the far right corner of the first room inside.
Snagging it completes the location- fast travel out once you've got it.