Like many believers, Cluff is convinced by the accounts of others who claim to have already found and even visited the “inner Earth”. Chief among them is Karl Unger, a German sailor said to be part of a 1943 U-boat expedition to the South Pole. The submarine apparently entered the Hollow Earth through an underwater passageway, and its crew were greeted by an advanced civilisation in a place called “Rainbow Island”. (Hitler was reportedly a believer, and some conspiracy theorists are convinced he escaped to the Hollow Earth at the end of the Second World War and is still there.) Then there’s Admiral Richard Byrd, a highly decorated US Naval officer whose supposedly hushed-up “secret diary” of a 1947 expedition to the North Pole is believed to contain descriptions of a land full of lush lakes, greenery and woolly mammoths (Byrd was actually in the South Pole at the time). There’s even a retired colonel in the US Air Force, Billie Faye Woodward, who claims that he and his twin sister (both hermaphrodites) were born in the Hollow Earth.