I haven't focused much on Marvel Comics of the Silver Age much thus far. One of my favorite stories from Stan Lee and Company was The Sleeper Saga, which appeared in Tales of Suspense #72-74.
Tales of Suspense was an anthology magazine back then, featuring two separate characters, Iron Man and Captain America. Cap was a relatively recent addition to the Marvel Silver Age, having been discovered floating in a block of ice in Avengers #4, about a year and a half earlier. Of course, he had been a mainstay of Marvel (then known as Timely) back in the Golden Age, fighting the Nazis and especially a Nazi villain named the Red Skull.
When Stan decided to bring him back they couldn't decide at first what to do with him. Should they keep him in the World War II setting and have him be a retro hero? Or should they tell modern tales. In the first year or so they varied back and forth with several modern stories interspersed with his World War II reminiscences. Remember, World War II was a particularly popular topic with youngsters due to the influence of Hollywood blockbusters like The Great Escape and The Longest Day.
In The Sleeper Saga, they hit on an superb idea; Cap would appear in the present day, but he'd still be fighting the Nazis. The background for the story was that the Red Skull had accidentally killed himself late in World War II, when a hand grenade he threw bounced off Captain America's shield and landed near him. With his dying breath he vowed that he would have his revenge twenty years later.
Twenty years later was conveniently 1965, when the saga started with Tales of Suspense #72:
There were three sleepers; the one shown on the cover was only the first. Captain America remembers the Red Skull's warning and notices that today is exactly 20 years later. He has a list of names and locations that he took from the dying Red Skull, so he sets off in search of the people. Unfortunately he arrives too late, and the first sleeper has been awoken. Cap tries to battle it, but he's no match for the giant robot, and it goes off in search of the second sleeper.
The second sleeper turns out to be an incredible airplane, which looks like a flying stingray. When it meets up with the first sleeper, the two combine into a terrible flying robot. Cap notes that there is room on top of the airplane for yet a third sleeper. What will it be?
Cap realizes that the Red Skull's plan is to destroy the entire world by having the sleepers burrow down into the Artic ice and thus into the earth's core, then blow themselves up. In the end, Captain America sky-dives onto the combined sleepers and sabotages them with a blowtorch, so they explode where they can't do significant damage to the planet.
I believe there was another sleeper that came along later. The Red Skull would return before the end of the year, still alive despite his apparent death in ToS #72.