Source: Action Comics #272-273 (1961)
Type: Parallel developmentYears before Captain Kirk laid eyes on a reverse-Earth, or an Earth where the Roman Empire was still thriving, Supergirl discovered Terra, Earth's double. The same, but different. And it's all because of THIS high school general science class:
What's next? Intelligent design?! So Supergirl goes to the Fortress of Solitude to use the computer facilities to find not only HER double, but a double of the planet, as shown in this very strange panel where Supergirl and Superman seem to telepathically jabber at each other:
A mistake? Or just the Silver Age getting out of control? Anyway, the Earth DOES have a double (Terra) and Supergirl flies there in the hopes of proving to Superman she's ready to go public. What happens on Terra, as they say, stays on Terra. Don't call it the guinea pig planet, it doesn't like that. So it's Earth, but it's not. Bigger Florida (which contains Mount Everest, by the by), Eiffel Tower in America, Statue of Liberty holding a flag...
...the pyramids are in Japan, the system's Jupiter is all spotty, its Mars has a ring, bicycles have four wheels, pet elephants are tiny, rabbits and tigers are one and the same dangerous (and probably not-at-all endangered) animal. And "Macropolis" is defended by one Marvel Maid, Terra' first hero, who lives in the Fortress of Marvels up in orbit.
The key's on a radioactive asteroid, so would-be, rocket-powered thieves could only get in by contracting cancer. Despite having the same powers, Marvel Maid isn't a Kryptonian. Instead, she was bore-rocketed to the surface world from the doomed civilization at the center of Terra!
Found and adopted by simple country folks, she was raised as Lea Lindy, now cub reporter at a major Macropolitan newspaper edited by Perry Waite. Just then, Marvel Maid is called away to a prehistoric world to extinguishing a forest fire that is DEVOURING THE ENTIRE PLANET, so she leaves Supergirl in charge to pose as her. Every time she screws up though, it's always because of some insanely stupid "difference". For example, when trying to melt an iceberg, it bursts into flames because it's made of flammable sea salts. There's also a bit with a "sinking bridge" that is their version of our drawbridges. Fortunately, her bacon is saved each time by Marvel Man, Marvel Maid's cousin, who has to operate in secret until she allows him to go public. He, too, was saved from the collapsing underground, but his rocket stalled and he grew to adulthood in suspended animation. So when he turned up on the surface, he was really bad at doing stuff.
And because Terra requires its citizens to all have identification papers, Kent Clark was immediately sentenced to years in prison. It's the perfect cover, so long as the guards never catch you sneaking into or out of your secret tunnel. When a meteor strikes the Fortress, it starts to fall to Terra. Marvel Man will try to catch it, but mustn't be seen doing so. Supergirl turns a piece of coal into a diamond to distract a farm wife from the action. Except diamonds are like Kryptonite to the underworlders! In fact, the farm wife wasn't motivated by greed when she grabbed it, but by civic duty.
In the second chapter, they rebuild the crashed Fortress before Marvel Maid returns. She's impressed with Supergirl nonetheless and doesn't blame her for the mistakes. Again she leaves Supergirl in charge while she flies to Earth to see it for herself and perhaps intercede on her double's behalf. After accomplishing a super-feat admirably, Superman, thinking it's his cousin, is so impressed he unveils his plans to let Supergirl go public. Her own army of Supergirl robots, full access to the Fortress and a worldwide release of footage of her saving the day from the shadows for years. From secret to superstar in no time. The real Supergirl is touched, watching from the alt-Fortress' space monitor, that Superman indeed DID plan for her to one day succeed. Marvel Maid sets the record straight, tells him of Supergirl's feats, but also of her understandable mistake. Together, they watch from HIS space monitor as she saves the day on Terra, though not without bungling it a little (the sinking bridge). Ultimately, the episode shows Marvel Maid that Marvel Man is ready to go public, and she arranges a big circus appearance (it's what they did in those days). And does Superman do the same for Supergirl?
Her mistake was not reading every book in the Terran library before setting foot there. In other words, STAY IN SCHOOL, kids!