As a fan of mythology, conspiracy theories, historical texts, and fantasy writing of all types I really enjoyed this book. The book is an interesting albeit highly speculative exploration of different cultures views and understanding of creatures they believe to be living around them as the title suggests, in caverns, caves, and generally underground. Depending on what other books you enjoy you might really like this (those who like ufo literature, paranormal stuff, and secret societies) or find it ridiculous and even historically irresponsible (those who enjoy hard boiled books on traditional anthropology, scientific method, and the like. I must say however that for what it is it is a generally coherent and fascinating read....that is until the end. Towards the end of the book the author takes some major leaps of faith in trying to string together all the information into an overarching (and very detailed VERY paranoid) plot that is being carried out by the evil that lives underneath us. I felt that it was unnecessary to do this in that the information provided was enough for the reader to reach their own conclusions. In summation I will say if you like weird books that are well written and string together ancient mythology into wild stories- this book is for you. If you don't like speculative non-fiction or "odd ball" ideas then you will not like this book. As a side note: the author overuses the word "obviously" waaay too much. No it is not obvious that because people go missing sometimes that the reptilian under-lords are kidnapping people for their genetic material. Even if this is happening it just is not obvious, cmon' dude really....