Tens of thousands of flooded caves are scattered around the planet, but less than five percent of them have been explored. This is the hidden frontier of human exploration. All the world’s mountains have been climbed. All the seas and oceans have been crossed. Our planet has run out of secrets to reveal—except for those that lie waiting underground.

Cinematographer and marine biologist Jon Shaw is on a quest to unlock the secrets of some of the world’s most remote and beautiful underwater cave systems. From the crystalline waters of the Nullarbor’s Weebubbie Cave, the mysterious Mayan cenotes of Mexico’s Yucatan Peninsula to the freezing currents of New Zealand’s Pearse Resurgence we join Jon on his hunt for new lifeforms, geological secrets and the remnants of lost civilizations. As we dive deeper and deeper into the unknown we must depend on elaborate diving technology, fierce courage and fellow cave divers to ensure we return to the surface unscathed.