Here's why:
* No futuristic elements: The books are set in the present day or historical periods, with no fantastical technologies or concepts like time travel, space exploration, or alien encounters.
* Real-world settings: The books feature locations that exist in the real world, both present and historical, and the characters encounter real historical figures or events.
* Focus on mystery and adventure: The central plot revolves around a treasure hunt and uncovering historical secrets, driven by the characters' search for clues and solving puzzles.
While the books involve a lot of action and suspense, they rely on real-world situations and historical themes, not fantastical elements, which is a hallmark of science fiction.