Remember the epic scene near the climax of Ant-Man, this Summer's best blockbuster, in which Scott Lang (Paul Rudd, killing it) disappears into the Quantum Realm? Of course you do.
And if you were looking carefully enough, you might even have noticed an Easter egg which could, in theory, hold the key to the Avengers and the rest of Marvel's heroes coming together to defeat Thanos in Infinity War.
Let's start at the beginning...
The science-y bit
The Quantum Realm is an alternative dimensional space accessible only on the micro-molecular level, which explains why Scott finds himself trapped inside it, having shrunk to subatomic size after defeating Yellowjacket.
You can relive the scene in question right here...
Listen carefully to the words spoken by Hank Pym ringing in Scott's ears, specifically the line at 0:44: "As you shrink... for all eternity..."
Inside the Realm there's no concept of "time" the way we know it. There are four levels, each progressively more difficult to return from: microscopic, atomic, subatomic (dissolution from physical form into energy) and a final, unknown level which seems to be a reflection of the soul in some way. It's possible (or even probable) that you could be lost inside for "all eternity".
But perhaps there are hidden layers to Hank's words...
Lost for Eternity...
Eternity is not just a measure of time - it's also the name of one of the most powerful beings in the Marvel comics universe. As the physical incarnation of time itself, Eternity possesses the collective consciousness of every last living creature in the universe.
Now, let's take a look at this screengrab taken from the Realm sequence...
In the left image you see a shadowy blur which looks very much liked a winged creature, and what winged creatures do we know who may or may not be lost in the Realm? That's right: Janet Van Dyne, wife of Hank and mother of Hope.
What it all means
But how does Janet connect back to Eternity, the all-powerful being? Well, bearing in mind that time as a fixed/linear concept doesn't exist in the Quantum Realm, Janet - having been lost inside it for thirty years - could have spent the equivalent of tens, if not thousands of lifetimes in there. And so, considering the origins of Eternity are unknown, it's perfectly possible that Janet has either become Eternity, or he has taken her physical form.
This solves a couple of problems quite neatly: in Ant-Man and the Wasp (due in the Summer of 2018, conveniently just before The Avengers: Infinity War Part 2), Scott and Hope will presumably join forces to rescue Janet from limbo, and in bringing her back to Earth they will have inadvertently brought their best hope of defeating Thanos.
On top of that, if Eternity has taken Janet's physical form, it will bring a dash of tragedy to the Ant-Man sequel when Hope realizes that there's no chance of bringing her real mother back.
It all kind of makes sense in a crazy, twisted way, but if there's a problem with the theory, it's that it's almost too complicated to explain on screen in a way that will be satisfying and easily comprehensible to the audience.
Personally, I'd love the theory to play out. Eternity could be the Avengers' only hope of defeating Thanos once he accumulates all six Infinity Stones to complete his gauntlet, thus gaining absolute omnipotence.
But perhaps I'm just getting carried away. What do you think?