Zack Snyder's Justice League: 10 Biggest Reveals From The New Trailer

In 2013's Man of Steel, Clark Kent found a Kryptonian ship full of what looked like escape pods, though one of them was quite clearly open in the background. A tie-in comic appeared to confirmed that it was Supergirl who made a home for herself on Earth thousands of years before Kal-El's arrival, with one fan theory suggesting that she might be the "first" Amazon.

Answering questions from fans, Snyder confirmed that Superman may not be Earth's only Kryptonian.

"That’s kind of an open-ended...if you study 'Man of Steel', and even in this film, there’s reference to the open sleeping créche which is, when they walk by it on the scout ship when they’re bringing Superman’s body into, bringing it back to life, there’s a sequence where they’re doing that through same hallway Kal-El went through when he first came into the scout ship." '

"And there’s a moment when he wipes the dust off one of the sleeping crèches and sees this, sort of, Kryptonian inside," Snyder continued. "Now, next to that crèche there’s an open crèche that just looks like someone clearly got out of that. Who that is remains a mystery to this day." Whether the filmmaker will get the chance to address that mystery is unclear.