A 1977 novel written by Ron Goulart and based on the comic series of the same name, Challengers of the Unknown follows a group of government-affiliated adventurers and paranormal investigators (Ace Morgan, Red Ryan, Rocky Davis, June Robbins, and Professor Haley) traveling to South America to investigate lake monster sightings. The monster is real and dangerous, but pales in comparison to another threat: a cabal of fugitive Nazi war criminals who have managed to stop their aging process and want to overthrow one of the few democratic governments in the region.
- Actually, I Am Him: When the team's captor asks if they've heard of the Nazi scientist Otto Wenzler, Rocky says that he died thirty years ago. The villain replies, "Not at all. I am Otto Wenzler."
- Argentina Is Nazi Land: The antagonists are a few hundred Nazis who fled to a South American country and want to conquer it for a new Reich.
- Famed In-Story: The team is made up of people who were celebrities even before surviving a harrowing plane crash and prevailing throughout a long series of dangerous and important missions on behalf of the government and various important individuals. The Dragon still gets an Underestimating Badassery speech about how their luck has to have run its course by now. It hasn't.
- Honey Trap: A woman who picks up Professor Haley turns out to be a tabloid reporter out to write a trashy article about the Challengers.
- Just Between You and Me: Downplayed. The villain tells the captive Challengers about his origins and immortality serum, but cannily refuses to tell them about his endgame, even if he does plan to kill his prisoners soon anyway. It makes no difference, as he dies of the side effects of the serum just a page later.
- Karma Houdini: The Nazis and some of their associates are punished, but the Mega-Corp conspiring with them to overthrow a democratic country and nationalize its oil remain unpunished and the Challengers' government contact has no interest in investigating their involvement.
- Not Me This Time: When the Challengers ask the Nazis how the lake monster factors into their plans, Wenzler reveals they had nothing to do with the monster's origin and are annoyed at how much unwanted attention it is attracting.
- Politically Incorrect Hero: The local president is an honest, reform-minded man, but he snidely says that women have no place in anything related to politics and he thanks the Challengers for not insisting that June be present when they talk important business.
- Shoot the Builder: The workers who built the Nazi bunker were shot and dumped in a trench they dug as part of the base, ensuring their permanent silence.