A spin-off of Countdown entitled, The Final Countdown.
This would still involve the famous big clock and "clock countdown" theme music, but instead of having a mere thirty seconds to come up with the longest word/a means of arriving at a randomly selected, specific number from a random assortment of letters/numbers, the contestants now have thirty minutes to diffuse a nuclear bomb that is set to destroy the entire Earth in a cataclysmic nuclear explosion before the timer runs out.