Tom Corbett Space Cadet #5

Tom Corbett and the space cadets become trapped on the moon when a rockslide cuts them off from their spaceship. After surviving beneath tons of moon rock with limited air, they are rescued and encounter a long-lost underground civilization of Lunarians—descendants of an ancient space expedition whose people have grown smaller over generations. When the cadets discover the Lunarians possess atomic fuel, they learn the aliens are actually smugglers planning to steal the fuel, leading to a confrontation where the cadets must prevent the theft and escape.