In this volume Sitchin compares Mesopotamia, Egypt and ancient American civilizations an comes to conclusion, that gods have visited America also. The reason for visit was simple - they have found precious metals like gold and copper, but they have also found tin, which has to be extracted from ores and gives hard bronze when mixed with copper. The sophisticated channels cut in the rocks were part of ore washing system. The resemblance of stories, buildings and myths suggests that behind names like Quetzalcoatl, Kukulcan and Viracocha stand the same deities we know from the first three volumes. The most impressive thing is that Americans didn't knew and use metals (except gold, of course), yet archaeologists have found stone blocks dressed and connected with bronze claps. And bronze must be obtained through a metallurgical process, which was surely not known nor to Mayas, Incas or Aztecs. Who needed tin from lake Titicaca? The answer is obvious.