Zeman's acclaimed prehistoric fantasy follows the adventures of four young boys who discover a mysterious cavern that leads them back through the early mammals to the age of dinosaurs. They encounter a wonderful gallery of stop-motion animated woolly mammoths, stegosaurs, pterodactyls, and other ancient creatures. Best of all is the boys' natural sense of wonder and scientific curiosity. The puppets Zeman used were based on the illustrations of Zdenek Burian. The film won the award for Best Children's Film at the Venice Film Festival in 1955.