As they search in the area where the ship was lost, Skydiver is attacked by the UFO and damaged, sinking to a ledge above the sea bed.
Two crew members reach the surface where Col. Freeman awaits them in an amphibious rescue aircraft but Lt. Tony Chin succumbs to his injuries on Skydiver.
They finally manage to escape the sub when SHADO divers blow Skydiver off the ledge and they can access unblocked missile tubes.
[4] Locations included: Neptune House at ATV Elstree Studios, Borehamwood; and Marlyn Cottage, Ley Hill, Buckinghamshire.
"[5] Describing "Sub-Smash" as a "strong Straker-centric episode", John Kenneth Muir praises the "understated nature" of the emotions on display and the use of Dutch angles to convey claustrophobia.