The X-Men may be known for protecting the world from mutant threats, but one of the team's most famous villains and occasional members isn't a mutant at all. The Juggernaut, Charles Xavier's stepbrother Cain Marko, acquired his super-strength through mystical means. Still, the Gem of Cyttorak made him one of the strongest and deadliest forces in the Marvel Universe.
But just because the Juggernaut's powers are magical doesn't mean that they're without explanation, and there's actually a surprising amount of comic book super-science behind the gargantuan body of the unstoppable Marvel villain.
1) The Juggernaut is Not a Mutant
Due to his personal connection to the X-Men's founder Charles Xavier, it's easy to assume that Juggernaut is simply a mutant. After all, Stan Lee first developed the idea for mutant powers as an easy way to dispense with lengthy origin stories. However, in Marko's first appearance in X-Men #12, Lee and Jack Kirby established the Juggernaut's mystical origins.
While serving alongside his stepbrother in the Korean War, Cain Marko entered a lost temple where he discovered the Crimson Gem of Cyttorak. Though the temple collapsed around him and Xavier thought him dead, the Gem actually imbued Marko with phenomenal power as the avatar of the dark otherworldly god Cyttorak. This means that many of the technologies that work against mutants - such as power dampening collars and Sentinel tracking devices - are ineffective against the Juggernaut. It also massively increased his human body to Hulk-like proportions, making him roughly 9.5 ft. tall and weighing just under two tons.
2) The Juggernaut is Virtually Unstoppable
Due to the mystical enchantments of the Gem of Cyttorak, the Juggernaut is physically unstoppable when he starts building momentum. Once he sets himself in motion, he can pummel through anything in his path while running at speeds up to 600 miles per hour. His immense strength and invulnerability make him a big enough threat even when he's not in motion, but this is the true power of the Juggernaut.
Additionally, the Juggernaut is able to produce a powerful force field around his body that has proven capable of withstanding the power of Thor and his enchanted hammer. All of this works together to make the Juggernaut virtually impossible to stop through physical means.
3) The Juggernaut Has Immense Strength
Cain Marko's body essentially acts as a conduit for the energy of Cyttorak. If the nigh-omnipotent being decides to cut Juggernaut off, he can reduce the flow of power to a fraction of its former strength, and Marko has had to get by with scraps of power that pales in comparison to what he once possessed. Cyttorak is a jealous and focused god of destruction, and he has punished Marko for accepting power from other beings or using his powers in the name of preservation rather than destruction.
Even when he is angry with his chosen vessel, Cyttorak's powers rarely leave Juggernaut's body entirely. Even when Marko is cut off he often retains an impressive degree of super strength and invulnerability that allows him to deal with most threats. Notably, Marko was strong enough to smash through the walls of reality itself when he became Trion Juggernaut.
4) Juggernaut's Body Maintains His Energy
To truly be unstoppable, Marko needs enough power to keep on going ad infinitum. For Juggernaut, his ceaseless reservoir of mystical energy serves as that power, meaning that he does not need sustenance in order to fuel himself.
Juggernaut has gone weeks and months without eating, sleeping or even breathing. In Amazing Spider-Man #230, Marvel's most iconic webslinger once famously fended off the raging supervillain by tricking him into a pool of wet cement. When Juggernaut could not gain the leverage needed to free himself, he simply sat and waited until the opportunity to escape presented itself much later.
5) The Juggernaut Still Has Weaknesses
Even if Juggernaut is famously "unstoppable," that doesn't mean he can't ever be defeated. Over the years, the most tried and true method of putting Marko down is to attack him psychically. The best way to do this is to remove his helmet, which serves as protection against psychic attack. However, even without his helmet Juggernaut still has protection from mental onslaught, and it takes a powerful telepath like Professor X or Jean Grey to fell him even without his helmet in place.
The X-Men's most common way of dealing with the Juggernaut involves the team working together to remove his helmet while Xavier or another telepath attacks him mentally. Over time, this has become more difficult for the mutants as Marko has learned to guard against this form of attack. While the Juggernaut might be one of Marvel's most physically imposing villains, this goes to show that even characters with extraordinary amounts of power still have their limits.