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 he, along with his geologist friend Dr. Tresselt, see an opportunity to use the device in the cave. The two adults, plus Danny, Joe, and Irene, enter the cave on an expedition. What they find is astonishing...but will they be able to escape to reveal their discovery?