Cryptoterrestrial hypothesis

The cryptoterrestrial hypothesis, also referred to as intraterrestrial,^[1] or inner-earth,^[2] proposes that unidentified flying objects are a sign of a technologically advanced population living on Earth alongside humans.^[3][4][5] The idea has been around since the beginning of the 20th century, was associated with the Shaver mystery and Walter Siegmeister, and underwent several reformulations, most notoriously by American journalist John Keel, French scientist and ufologist Jacques Vallée and American novelist and blogger Mac Tonnies. It is often associated with the concept of a Hollow Earth and the Silurian hypothesis, however the three are not equivalent and other theories have been put forth to account for the UFOs’ nature and origin within this framework.

This hypothesis is coinsidered pseudoscientific in nature, just like the disciplines concerned with its study such as cryptozoology and ufology. Furthermore, in spite of its early popularity, the concept that UFOs could be indigenous to planet Earth is considered unconventional even amongst ufologists.^[6][7][8]