re: Alexander the Great
His sarcophagus was reportedly made of crystal so the body could be viewed. It was being transported back to Macedonia and was hijacked by soldiers serving Ptolemy, the general and future dynast of Egypt. It was transported to Alexandria Egypt and was apparently available for viewing for many years. Some speculate the sarcophagus is somewhere out there in the sea fronting the city.. so is Cleopatra’s. The sea level is so much higher now that most of ancient Alexandria is out there in the sea. Why the tomb wasn’t treasured and preserved is lost to time.. there is no record of what became of it.
I’m pretty sure an earthquake dropped it below sea level. The sea did not get higher, the ground sank down
late 4th century religious unrest. you are surely aware of the speculation about st. mark’s remains.