Forbidden Island

From the introduction:

Somewhere out in the uncharted waters of the Bermuda Triangle lies a small isolated island. This island has stayed largely undisturbed for millions of years. Dinosaurs and primitive mammals still roam its plains and jungles, prehistoric man seeks shelter in the cooled vents of its volcanoes. Unknown to its inhabitants and the outside world an unknown race constructed a great force field, to keep out both intruders and the harshest weather, powered by the heat of the islands volcanoes it still keeps silent duty in the caves under the island. Here lies adventure.