Cryptozoology
Grumley was interested in cryptozoology, he was the author of a book on Bigfoot, titled There are Giants in the Earth the book was first published in 1975 with a later edition appearing in 1976. In the book Grumley concluded that anthropoid giants once roamed the earth, and that today there are still isolated survivors which he claimed are living in underground tunnels and caves.
Read more about this topic: Michael Grumley