There has long been rumors of a vast underground city under the Aztec capitol. J. Frank Dobie wrote of it in his book APACHE GOLD AND YAQUI SILVER.
LEW WALLACE, who wrote BEN HUR, also wrote a novel of the Spanish in Mexico, THE FAIR GOD in which the underground city plays a part.
This book was supposed to be made into a movie back around 1964 by Warner Brothers. It was never made.
Teotehuacan, as you may know, predates the Aztecs by hundreds of years. It was so old that even the Aztecs were clueless as to who built it. The Aztecs get all of the credit for the Mexican civilizations, even though they were latecomers to central Mexico.
I visited Teotehuacan some 40 years ago. Still have some sharp obsidian “razor blades” I dug out of one of the unrestored mounds. The best part was climbing the two big pyramids, huffing and puffing to find a ten year old kid selling cold bottles of Coke. Then realizing he had hauled up the soda and ice himself.