Voyage to the Bottom of the Sea (1964)

S1E2 The City Beneath the Sea

While looking into the vanishing research vessels in the Aegean, the Seaview sustains damage from an explosion. Captain Crane goes on land, posing as a freelance diver. He encounters a former diver who, after a tragic incident, can no longer speak and insists he has witnessed a "city beneath the sea." Crane ends up being hired by the antagonist, but his true identity is quickly revealed. Captured, he faces peril and comes close to losing his life.