The Incredible Petrified World (1959) starring John Carradine, Robert Clarke, Phyllis Coates

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Trapped at the bottom of hostile waters, explorers discover their only chance for survival is a labyrinth of undersea caves. The caverns provide oxygen and shelter for the survivors, but lack an escape route to the surface. When they come across a human who has lived in the tunnels for fourteen years, their hopes for a passage out are dashed. Facing an eternity spent in the caverns of the deep, the four explorers’ relationships deteriorate. As their confidence fades, their seaside leader refuses to give up on the team and risks everything to save them.