A Long Drink Papyrus Riddle
This riddle can be found in the House of Life, a ruined temple south of Siwa village (and part of the Healer Side Quest, if you've already accepted it). The papyrus itself is unrolled on table beneath a tent past the Temple gates.
The riddle reads "In Siwa, come find me at the bottom of the only bowl big enough for a god." It is clearly referring to a well, because, as we learned in Zork, all good riddles refer to wells. In this case, the well is the round indentation on your map, south-east-south of the Temple of Amun.
Dive in and loot the sarcophagus for some Legendary gear.
The riddle of why there's a sarcophagus in the drinking water will have to wait until another time.