Throughout our planet there are geologically very old artifacts (millions of years old) that confirm the existence of an unknown civilization. She had the habit of building underground cave complexes, in which the millions of inhabitants of an unknown, perhaps humanoid species, apparently could live.

Although these complexes are deployed on several locations, they seem to be interconnected and there will surely be much more than they have so far been officially discovered or discovered.

Their characteristic features are: perfect acoustics, monolithic structures, unknown tunneling technology.