The Infinity Stones are the most powerful force in the Marvel Cinematic Universe. For 10 years now, they've been the ultimate MacGuffins; they've been the key plot device in everything from Captain America: The First Avenger to Thor: The Dark World. Now, in Avengers: Infinity War, the buildup is coming to a head. Thanos is headed for Earth, seeking the unlimited power of the Infinity Stones. His goal is to "rebalance the universe" by literally wiping out half the cosmos's life with a snap of his fingers.Fans had assumed the last few movies would reveal the final Infinity Stone, the Soul Stone. To their surprise, they didn't; popular fan-theories have been left unresolved, and fans are increasingly puzzled and confused. In the wake of the full Infinity War trailer, the Russo brothers took to Twitter to troll fans. "Where is the Soul Stone?," they asked, knowing full well how the tweet would be received.RELATED: Avengers Infinity War Trailer: Biggest Secrets & Things You MissedThe Soul Stone is a mystery in-universe, as well. According to the Avengers: Infinity War Prelude, it's been missing for millennia, and even the Masters of the Mystic Arts don't know where it is. It's precise location is a mystery; one that the trailer may have subtly hinted at, though. And maybe the question of where it's located is the wrong question altogether.This Page: Where is the Soul Stone?

Where Is The Soul Stone?

The last couple of years have seen two prominent theories as to the Soul Stone's location. The first is that Heimdall has it; after all, he has the power to see "every soul" in the Nine Realms, and his powers are associated with the color orange. Fans were convinced Heimdall would reveal the Soul Stone in last year's Thor: Ragnarok - but he didn't. In fact, the film didn't offer any hints to support the Heimdall theory. What's more, Ragnarok treated the Asgardians differently to the other Thor films, accepting that they were "gods" and thus removing the need to explain any powers at all. The Heimdall theory still has some supporters - there's evidence Heimdall will die in Infinity War, and some argue that's because Thanos takes the Soul Stone from him - but it seems a lot less likely.

Another common theory is that the Soul Stone was in the Vibranium asteroid that crashed to Earth in Wakanda, 2.5 million years ago. The trailers for Black Panther had teased a mystical aspect to the film, which ultimately turned out to be the "Ancestral Plane." Again, though, Black Panther came and went. Director Ryan Coogler admitted he felt the Soul Stone was unnecessary and insisted there was never a push to put an Infinity Stone in his movie. Marvel had other plans for the Soul Stone.

And yet, the Soul Stone is clearly important to Thanos's plans. No matter the power of the other Infinity Stones, he will only achieve his goal if he gathers all six into the Infinity Gauntlet. Meanwhile, Marvel's Avengers: Infinity War Prelude reminded fans about the Soul Stone; it also upped the ante a little by describing it as the most dangerous Infinity Stone of all. The Russos' tweet is deliberately designed to fan the flames of speculation as well. This last Infinity Stone is clearly important.

RELATED: Who In The MCU Knows About The Infinity Stones?

Perhaps the most interesting detail of all is the fact that Thanos doesn't actually seem to be looking for the Soul Stone. It's as though he already knows where he can find it.

Infinity War Is Heading To Titan

In a set visit last year, the Russo brothers told Screen Rant that the Infinity Stones provide a "structure" for Infinity War. With the release of the full-length trailer, that structure is becoming fairly clear, but there is one puzzling mystery. The film seems to feature a number of scenes set on an alien world, one with a distinctive orange hue; Thanos will seemingly demonstrate his power on this planet, literally tearing a moon out of orbit.

Fans initially believed this world was actually Xandar, ravaged after Thanos attacked in pursuit of the Power Stone. Marvel told Entertainment Weekly that wasn't the case; instead, they identified it as Titan, "the ruined homeworld of Thanos." Footage presented at D23 2017 suggested that a number of the Avengers would actually wind up on this world, battling the Mad Titan as he tosses a moon at them. So clearly, for some reason, Thanos will first arrive on Earth, confront some of the Avengers, and the head to Titan. Why would he do this? The Russos' comment implies that this is still somehow tied to Thanos's quest for the Infinity Stones.

Soul Stone Potential Locations in the MCU

The Soul Stone Has Been Lost For A Long Time

The key, perhaps, is that the Soul Stone has been lost for millennia. While all the other Infinity Stones have been unearthed, nobody seems to know the Soul Stone's current location. But that doesn't necessarily make the Soul Stone impossible to find; all you need to do is first learn where it was, and then acquire the Time Stone to go get it. That would fit with the structure of the film; Thanos first heads to Earth to take the Time Stone from Doctor Strange. He then combines the power of the Time and Space Stones to open a portal to the last known location of the Soul Stone, and goes to claim it. Should he be successful - or, alternatively, should the Avengers instead retrieve the Soul Stone before returning to Earth and Wakanda - that would explain why the Soul Stone has been missing for so long. It was taken out of time.

RELATED: How Avengers 4 Is and Isn't Part 2 of Infinity War

Meanwhile, this would potentially flesh out Thanos's connection to the Infinity Stones. His homeworld has been described as "ruined;" but wouldn't it be ironic if Thanos himself was the one who ruined it, when he stepped back in time to claim an Infinity Stone? It would also explain how Thanos first learned of the Stones - he grow up on a world that possessed one.

Not only does this idea fit rather well, it also establishes a thematic tie between Infinity War and Avengers 4. By now, it's clear the fourth Avengers movie will be a full-on time-travel film. A succession of set photos have teased that the Avengers are attempting to rewrite history, although possibly not using the Time Stone; instead, given strong ties to Ant-Man & the Wasp and Captain Marvel, it's likely the heroes will be travelling via the Quantum Realm. In thematic terms, using time-travel in Infinity War would help to set that up; it would introduce the idea of actual travel through time and space, a logical progression from Doctor Strange. The Marvel Cinematic Universe has always grown through incremental steps, and this would certainly fit that pattern.

Right now, of course, this is only another (admittedly complex) theory. The Soul Stone's location remains a mystery, and only one fact can be said for certain; the Soul Stone must appear in Infinity War. The question that has tantalized Marvel fans for years now must finally be resolved, as Thanos pursued his insane quest to acquire the power of the Infinity Gauntlet.

MORE: Where The Hell Is The Soul Stone?

Key Release Dates