The Lost City of the Aztecs

Four chums find a secret cipher stuck inside the binding of an old book written many years ago by a famous geologist. After repeated efforts the boys solve the code, learn of the existence of the remnant of a civilized Aztec trip, living inside an extinct crater in the southern part of Arizona. How they find these Aztecs, and their many stirring adventures makes a story of tremendous present day scientific interest that every boy will enjoy.