Please don't say case closed. I am all for the idea there was one or more scientifically advanced civilizations that got wasted somehow. I only mention granite's possible weight perhaps being like Styrofoam. What has the advancement of time been able to do to materials? Petrified wood turning to stone for example. I wasn't there when the walls were built so I have to consider what could have been available to the builders. My first thought was the materials used and what if the materials changed over time from what they were. We live today with the knowledge materials will eventually fall apart with time. What materials may there be that just become more able to resist continuous bombardment of the elements? Carbon becomes diamond. What of materials turned into nano size that exhibit extreme changes in strength, electrical conductivity, thermal conductivity?