Iziah Brown and his family, who lived nearby, had discovered the fountain. They kept it secret for a while. The water of the fountain revived some of the dinosaur fossils, bringing them back to life. They made their way out of the cavern, and attacked the people of Eternity.
When Peri Brown visited the Brown family with Josh Larson, Iziah and his wives took them to see the fountain. The Master also showed up, having recently escaped from Sarn. He drew a bucket of water from the fountain, but Peri slammed into him, causing him to spill the water. Wherever the water touched a fossil, it brought the dinosaur back to life. The humans escaped the cavern, and the Master hypnotised the dinosaurs and left the cavern.
Later, after the Master and the dinosaurs had been defeated, the Fifth Doctor used dynamite to close the cavern, sealing the fountain in. He saw it as a temporal freak that reversed the effects of time. (PROSE: A Town Called Eternity)
The storyline of the 1957 science fiction B-movie Swamp of Horrors involved Professor Derek Spire and Melody Malone searching for the Fountain of Youth in the Louisiana bayou. (PROSE: Swamp of Horrors (1957) - Viewing Notes)