Danny Dunn and the Fossil Cave

Danny and his friend Joe Pearson discover the entrance to a cave in the woods near their home. Professor Bulfinch has just invented a portable x-ray machine, and the Professor, along with his geologist friend Dr. Tresselt, sees an opportunity to use the device inside the cave. The two adults, along with Danny, Joe, and Irene, enter the cave on an expedition. They make an astonishing discovery, but they encounter a significant problem, which prevents them from leaving the cave...
Danny Dunn and the Fossil Cave is the sixth novel in the Danny Dunn series of juvenile science fiction/adventure books written by Raymond Abrashkin and Jay Williams.