The world of Doc Savage is a very deep and ever-thriling mine.

Last night I finished reading The Giggling Ghosts (1938), a decidedly minor adventure in the series, but it had interesting facts in it about the construction of the Lincoln Tunnel and the World’s Fair. The plot revolved around an elaborate criminal real estate scheme to defraud, depopulate, and then buy up all the land surrounding the tunnel mouth in New Jersey. It got me researching into this and the New York City World’s Fair of 1939.
After initially thinking that the tunnel mentioned in Ghosts was the Holland Tunnel, I figured out which tunnel was meant and discovered that construction on the Lincoln Tunnel’s second tube had indeed been halted in 1938, though this was due to materials shortages in the build-up to World War II. This was the factual kernel for the tale.
I discovered that Ghosts was the first mention of the then under construction World’s Fair. Also, research from Will Murray showed how the World’s Fair President Grover A. Whalen approached Doc Savage’s publishing house Street & Smith with a promotional deal. Since Doc Savage was the most popular character of the Pulp Era and a scientific detective who solved unfathomable mysteries featuring a host of amazing high tech gadgets, the character was seen as the perfect match for the Fair’s theme of the World of Tomorrow. So, after taking a private, fact-finding tour of the World’s Fair construction site, Lester Dent and William Bogart wrote The World’s Fair Goblin in the fall of 1938.
In the Nostalgia Ventures reprint (Doc Savage, volume 17) article, Murray included a note from Street & Smith editor, John Nanovic, with this excerpt:
Armed with plenty of little note cards on which to take notes; cameras with which to take pictures; and permission to go through every nook and corner of the Fair grounds, we set out with Kenneth Robeson [authorial housename] to look over the possibilities. We were given beautiful and extensive maps of the entire grounds; were hoisted high up inside the Trylon and taken inside the Perisphere. The Perisphere is a huge ball which will seemingly be floating in water. The Trylon is a three-corner shaft, coming to a point almost eight hundred feet in the air. … Anyway, there it was, and there we were, and right in the midst of our trip, one of the steel workers on the Trylon, in passing comment to us, said, “This should be a honey of a place for Doc Savage to get into!”
It sure was. The original draft of Goblin was called The Man of Tomorrow, but it was deemed better to change the title in order to prominently promote the World’s Fair name. It was released with much fanfare in the spring of 1939 to coincide with the Grand Opening of the Fair.

I just finished posting some historical notes on the Wikipedia articles about these findings, including this later Doc book The World’s Fair Goblin.
I love the fact that even a light-weight Doc Savage adventure is a bottomless mine of information and research when you get into it. I’ve learned more strange yet fascinating details about science, technology, history, chemistry, anthropology, archaeology, architecture, languages, cultures, etc., through reading Doc Savage than any other single source in my life. It is rather amazing.