Tom Swift in the City of Gold, or, or, Marvelous Adventures Underground, is Volume 11 in the original Tom Swift novel series published by Grosset & Dunlap.
Tom receives a message from his missionary friend whom he saved from captivity in Africa during the adventures of the preceding volume. The message describes a wonderful underground city, filled with treasures of gold, somewhere deep in the heart of Mexico. Not one to turn down adventure, Tom accepts the challenge to find the lost city.
Around this time, Andy Foger and his father had lost their fortunes and are off after Tom's trail in order to steal the treasures from him. In order to make the trip possible, Tom must remodel his previous airship- a hot air balloon with an enclosed cabin.