'Subterranean Sea' by Alan Fennell

When the Stingray crew has their holiday abruptly cancelled, they set off to probe the ocean depths. Troy and his friends soon discover a deep undersea shaft, which leads them to a desert-like plateau; however, while they are investigating their surroundings a huge wall of water appears and transforms the desert into an underground sea. As the waters threaten to drown them, Troy and Phones desperately swim against the tide - but will they reach Stingray in time..?

Don Mason (Captain Troy Tempest), Robert Easton (Lieutenant George Lee ‘Phones’ Sheridan / Agent X20), Lois Maxwell (Lieutenant Atlanta Shore), Ray Barrett (Commander Sam Shore / Lieutenant Fisher / Titan)

Directed by Desmond Saunders
Produced by Gerry Anderson

TX (UK): 25th October 1964

Notes:
*Featuring Captain Troy Tempest, Lieutenant George Lee 'Phones' Sheridan, Commander Sam Shore, Lieutenant Atlanta Shore, Lieutenant Fisher and Marina