Tom Swift in the City of Gold, or, or, Marvelous Adventures Underground, is Volume 11 in the original Tom Swift series. Tom receives a message from his missionary friends, those 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 soon builds another airship and is off with his friends to find the underground city. Tom Swift has gone through four series and through generations of the Tom Swift family. The original Tom Swift series is referred to as Tom Swift Sr. Many characters reoccur through the series including: Mary Nestor, who Tom eventually marries, Mr. Damon, who is always blessing almost everything, Eradicate Sampson, who is braver and more intelligent than assumed, Eradicate"s mule Boomerang, and many others. Tom Swift is a talented mechanic with a special love for airships and airplanes.