Stingray - Season 1 Episode 4 Subterranean Sea

When their vacation is cancelled, the Stingray crew probes into the ocean depths, They discover a deep shaft that leads to desert-like plateaus. Before they can complete their investigation, a wall of water turns the desert into an underground sea. Swimming desperately back to Stingray they attempt to relocate the shaft before their oxygen runs out. Out of desperation they enter the first shaft they see. They surface near a beautiful tropical island. The perfect place for a holiday