Ancient Underground Cities and Tunnels

My previous books on ancient civilizations, megaliths, strange objects out of time, Atlantis, and Giants have led me to this last book on underground structures.
This book is about ancient tunnels and underground cities. From the previous research I’ve done on related topics I do strongly believe that these findings are related to ancient civilizations which we know existed from ruins on and under the Earth.
Some of these findings are stories which need more evidence to prove they are real th...