Fox Mulder suspected that the creatures had evolved over centuries to gain the ability to manipulate light so that they could hide extremely well in broad daylight. He also suspected that there was a connection between them and the Spanish explorer Ponce de Leon, who had visited the area seeking the Fountain of Youth centuries earlier. Scully also discovered a carved pole in the creatures' underground lair with the phrase "Ad Noctum" carved in it. The phrase translated as "into darkness" and had been routinely carved on posts which the Spanish Conquistadors had lashed American natives to as a warning.
In 2001, Leyla Harrison and John Doggett suddenly became trapped underground. While considering how they had become trapped, Leyla Harrison remembered that Mulder and Scully had once investigated a case in Florida that involved subterranean men who took people underground, referring to the invisible creatures. However, John Doggett did not believe that he and Leyla Harrison had been trapped by a man because Doggett was aware that they had fallen through some kind of trap, which he speculated may have been created to provide live meals for a creature dwelling underground. (TXF: "Alone")