It ignited curiosity in the man. He desperately wanted to know what that spot meant. So he decided to solve the mystery. He began with hammering one of the marks but surprisingly, the mark sounded hollow. It fired up his curiosity even more. So, he decided to drill into the mark and find out what it really is. He said “I, er, noticed that there were these things down here… circular, whatever.” He had not expected what he was going to come across after completing the drilling process. What you think could be this circular mark hiding?