Marvel’s highly anticipated upcoming movie is a culmination of the past ten years for the Marvel universe. From 2008 with the debut of "Iron Man," to the record-shattering "Black Panther" in 2018, Marvel Studios has never disappointed fans.

As we approach the highly anticipated culmination of the past ten years for Marvel Studios, "Avengers: Infinity War," we're still missing one of the most important pieces: the soul stone.

One of the six infinity stones in the Marvel universe, the soul stone is perhaps the strongest amongst them. Created during the time of the creation of the universe, six singularities were put into the infinity stones: time, space, power, reality, mind, and soul. Throughout the past 10 years, we’ve seen just about every stone— yet the soul stone remains a mystery. Fans have speculated some incredibly detailed and extremely unreasonable theories. Here's what I believe to be the five most reliable theories— with a bonus fifth, just because I think it would be the coolest.

1. Wakanda

The theory is that the stone lies at the heart of the Vibranium meteor that hit the nation years ago and imbued the ground with the powers of the heart-shaped herb. This checks out because as we saw in "Black Panther" after taking the heart-shaped herb, T’challa went to the ancestral plane and was able to communicate with the souls of his ancestors, which lines up almost perfectly in terms of the soul stones capabilities. This theory also lines up with the infamous T.H.A.N.O.S. theory which states that each infinity stone is stored in an object with the first letter of each object spelling Thanos. For the soul, the stone would be in the heart-shaped herb.

However, this theory is doubtful because recently, the director of "Black Panther," Ryan Coogler along with the Russo brothers claimed that the soul stone is in fact not in Wakanda. We also saw at the end of "Thor: Dark World" that it is unsafe to keep two infinity stones close together; and if there are already two stones on earth, time and mind, it would be unlikely to have the third on earth as well. Although some disprove this by stating that the soul stone is deep in Wakanda and has been there long before the other stones came remotely close.

2. Heimdall

While absurd on some levels, the Heimdal theory adds up in more ways than one. For those unfamiliar with Heimdal, he is the all-seeing guardian of the Bifrost from Thor. It might be a coincidence, but the fact that Heimdal's powers in the Thor movies have been revealed as him being able to see "every soul" in the universe has a particularly noticeable resonance. We also know that the soul stone is orange, matching the exact color of Heimdal’s eyes, and in Thor's vision in "Age of Ultron," Heimdal is blind. Thanos could have taken the stone from his eyes. Not to mention this theory ties in perfectly with the infamous T.H.A.N.O.S. theory.

3. Adam Warlock

If Marvel Studios does decide to follow through with the infinity gauntlet comics, we expect Adam Warlock to have the soul stone. Introduced in the post-credit scene of "Guardians of The Galaxy Vol. 2," it would logically make sense for Adam Warlock to have the soul stone. Given to Adam by a being known as the High Evolutionary, Adam was the one to put out the initial rallying cry to bring the Avengers together to stop Thanos from taking all six infinity stones. However, it seems unlikely that Adam Warlock will play such a large part in the "Infinity War," especially since Adam’s creation was such an afterthought.

This theory also follows the T.H.A.N.O.S. theory as Adam’s original name was H.I.M. However, it is extremely unlikely as the Russo brothers and Guardians director, James Gunn, definitively stated that Adam Warlock will not appear in "Infinity War."

4. Thanos

We know that Thanos gave Loki his scepter in the first "Avengers" installment, which contained the mind stone. Some are speculating how ludicrous it would be to give away one of the most powerful pieces in the Marvel Universe, the mind stone. The theory is that Thanos gave Loki the mind stone because Thanos already has an infinity stone, the soul stone. While in the trailers we don’t see the soul stone on the infinity gauntlet, it could easily have been edited out for trailer purposes.

5. Tony Stark

This final theory seems highly unlikely but is most definitely my favorite. New promo images suggest that "Iron Man" is somehow connected to the soul stone. In Howard Stark’s video message to Tony Stark in "Iron Man 2," Howard reminds his son that “What is and always will be my greatest creation is you.” We also know that the Marvel Cinematic Universe, or MCU, started with and has been revolved around Tony Stark since the release of "Iron Man" in 2008. So it wouldn’t be insane to speculate he somehow has a connection to the most powerful stone in the MCU.

As much as fans can theorize, we will have to wait until Thanos arrives in "Avengers: Infinity War" on April 27th to know for certain.