An underground dinosaur cave is located at the Earth's core in the 1987-1996 series. The Turtles discover it in the season 3 episode "Turtles at the Earth's Core" when a dinosaur has broken through the crust and into New York City. In the cave live various prehistoric animals, who have been separated from the echosystem above for millions of years.
Sunlight can't reach into the cave. Instead, life there relies on energy coming from a strange crystal atop a tall mountain. Krang and Shredder want it to repower the Technodrome, and the Turtles must stop them.