241. "Superman: The Super-Key to Fort Superman", "Congo Bill: The Great Jungle Hoax", "Tommy Tomorrow: The Strangest Crew in the Universe"

Whenever he needs to get away from the day-to-day acts of heroism, Superman retreats to his hidden Arctic Fortress of Solitude. Access to the Fortress is gained by way of a giant metal key – a key so heavy, that only Superman could ever hope to lift it.