From the very beginning, the Marvel Cinematic Universe has revolved around the Infinity Stones -- even if characters didn't know it. The powerful gems that comprise the Infinity Gauntlet will finally come together in The Avengers: Infinity War, but the Soul Stone has yet to be discovered in the MCU. Theories about where and when the powerful artifact will debut have circulated for quite some time, but one of the latest is also one of the most intriguing. Specifically, this theory posits that Odin has been hiding the Soul Stone under his eyepatch this whole time, and it will play a significant role in the events of Thor: Ragnarok.
According to this theory, Odin has been in possession of the Soul Stone for the entirety of his tenure as the King of Asgard, and it has been secretly hiding under his eyepatch (gross) the whole time. Hela (who has been banished from Asgard at the time of Ragnarok's opening) eventually finds him in New York (similarly banished there due to Loki's treachery) and steals it for herself -- possibly even killing Odin in the process. The acquisition of the stone will imbue Hela with unprecedented power over all souls in The Nine Realms, essentially turning her into the "Goddess of Death" when she reaches peak power.
It makes quite a bit of sense, and it also helps explain the makeover she gets when she steals the stone -- which may have previously been in her possession before Odin took it.
The Thor corner of the Marvel mythos seems like a reasonable place to find the Soul Stone when we consider how the concept of "souls" plays into many Thor characters. For the titular hero, the Soul Stone could very easily be what imbues Mjolnir with its powers, thus explaining how Hela can overcome the hammer and destroy it during their first encounter. Similarly, Heimdall's power is the ability to "see" every soul in the cosmos through The Odin Force, and the fact that he is blind in Thor's Ragnarok vision (seen in The Avengers: Age of Ultron) could signal that the theft of the stone will rob him of his power.
Oddly enough, this wouldn't even be the first time that a key plot point has been hidden underneath an eyepatch in the Marvel Cinematic Universe. In Captain America: The Winter Soldier, we learned that Nick Fury secretly kept an alternate retinal scan for the S.H.I.E.L.D. mainframe hidden under his iconic patch. It's unclear if the coincidence is intentional, but it shows that plot points in the MCU have a habit of echoing each other over time.
This theory (which initially surfaced on Reddit) is surprisingly airtight, although there are certain issues with it that the MCU will have to deal with. Specifically, in Thor: The Dark World The Warriors Three made a point to say that keeping two Infinity Stones in the same realm is unwise. This raises questions about why Odin would allow any other stones to enter Asgard with the knowledge that he already has one in his eye socket, but it doesn't change the fact that many of the other details of this theory could hold up under scrutiny.
Whether or not Odin is hiding the Soul Stone under his eyepatch remains to be seen, but we will know for certain once Thor: Ragnarok debuts in theaters on November 3, 2017. From there, the God of Thunder will go on to reunite with the rest of the Marvel Cinematic Universe heroes when The Avengers: Infinity War debuts next year on May 4, 2018.