The history of adamantium within the Marvel Universe is a bit tricky. We know that it goes back to the days of World War II, when scientist Myron MacLain was working on creating an indestructible metal and ended up creating what we now refer to as "proto-adamantium," but it most commonly just called "Captain America's shield." The small piece of metal has many properties of adamantium, but is actually even stronger.
True adamantium is supposed to be just a bit less unbreakable than proto-adamantium. However, true adamantium has been broken so many times that Marvel introduced the concept of "secondary adamantium," which is essentially saying "Any adamantium that you saw get broken wasn't actually true adamantium." True or not, here are 15 characters that have broken some form of adamantium, whether it be proto-adamantium (Cap's shield), true adamantium, secondary adamantium or whatever.
For an "unbreakable" metal, it's interesting that in its very first official appearance in the pages of "Avengers" #66 (by Roy Thomas, Barry Windsor-Smith and Syd Shores), Thor does ever so slightly dent it with his hammer, so it suggests that Thor could break it under the right circumstances, like if he just kept pounding on it over and over again.
It was actually an instance of Thor destroying a version of Ultron made out of adamantium during the crossover, "Ultron Unlimited," that introduced the concept of secondary adamantium into the Marvel Universe (it had been theorized in handbooks before, but not yet in the actual comic books), as Iron Man notes that Thor wouldn't have been able to destroy Ultron had he been made out of "true" adamantium. Thor, of course, questions the validity of Iron Man's "science," as he remains confident that he could destroy adamantium if he ever really needed to do so.
One of the problems in a shared comic book universe of ever making any sort of definitive statement (whether it be "this metal is unbreakable" or "this person is the greatest martial artist in the universe") is that you are always just subject to the next writer down the line who feels like writing something different. Along those lines comes "Marvel Team-Up" #18 (by Len Wein, Gil Kane and Mike Esposito), which took the idea of "unbreakable" adamantium and essentially turned it into a joke.
The concept behind the issue is that there is an entire factory made out of adamantium, which is already hard to believe, since adamantium is so rare. Even Apocalypse just took the adamantium out of Sabretooth's body when he gave it to Wolverine, rather than making some of his own. Then it went nuts when Blastaar, the Negative Zone villain who is pretty powerful (though never treated as a top of the line threat), uses his explosive powers to destroy the adamantium walls of the factory.
The trick with Hulk is that there is no real upper limit to just how strong he can get, as his strength increases along with his anger, so he could theoretically get to some amazingly high levels of strength. We saw some aspect of this during "World War Hulk," where Hulk practically destroyed the entire planet of Earth with his strength (he was at his peak anger during that crossover due to blaming the heroes of Earth for the death of his wife). So it seems likely that Hulk eventually would be strong enough to break adamantium.
However, for him to do so easily in defeating Blastaar in "Marvel Team-Up" #18 was probably a bit too much. In a notable "What If...?" comic where Hulk managed to kill Wolverine, he did so by being able to eventually snap an adamantium piece of Wolverine's vertebrae, therefore severing Wolverine's spine.
The first example of someone actually breaking adamantium in such a way that could not be later explained away by saying that it wasn't "true" adamantium came in "Uncanny X-Men" #160, when the X-Men ran afoul of the demon Belasco and his evil limbo dimension. The X-Men learned that they had already fought against Belasco (and lost) in an alternate timeline where they had been lost in Limbo for decades. Wolverine and Colossus were both eventually killed, with Nightcrawler being turned to Belasco's side and Storm becoming a powerful sorceress in her own right.
In any event, Belasco's pet demon, S'ym (named in reference to "Cerebus" creator Dave Sim), broke a piece of the dead Wolverine's adamantium skeleton off and used it as a toothpick. He then also stabbed Colossus with it. We suppose you could argue that adamantium is somehow more vulnerable in limbo or that magic has a special effect on adamantium.
11 DOCTOR DOOM
During the year-long maxiseries, "Marvel Super Heroes Secret Wars" (by Jim Shooter, Mike Zeck and John Beatty), a group of the most prominent superheroes and supervillains were each brought to a mysterious Battleworld by a powerful being known as the Beyonder, who ordered them to all fight against each other for his amusement. While the heroes and villains then went forth and battled against each other (mostly because the villains decided to attack the heroes), Doctor Doom had another plan.
He decided to steal the power of the Beyonder for himself, and ended up actually succeeding. He then went to the heroes (who had successfully defeated the villains by this point) and offered to let them live if they agreed to serve him. They said no, so Doctor Doom struck them down with a gigantic bolt of energy. The bolt of energy was so powerful that it actually broke Captain America's shield! Luckily, once Doom was defeated and the Beyonder's energy returned, a bit of the Beyonder's power was left in the hands of the heroes and Cap used it to fix his shield.
A few years later, a different villain also gained ultimate power. In the miniseries event, "Infinity Gauntlet" (by Jim Starlin, George Perez, Ron Lim and Joe Rubinstein), the mad Titan known as Thanos finally got his ultimate desire and brought together the Infinity Gems to create the Infinity Gauntlet, which allowed him control over reality itself. In his first act as the new "god," Thanos snapped his fingers and killed half of the people in the universe.
The surviving superheroes teamed up to attack Thanos as a distraction while Silver Surfer was poised to swoop in and steal the gauntlet. The heroes were all slaughtered one by one until there was only Captain America left. He bravely stepped right up to Thanos, buying Surfer time. Thanos destroyed Captain America's shield, but Cap still just stood there. Sadly, the Surfer missed and Thanos killed Cap with a backhand slap. Luckily, Adam Warlock eventually got the Gauntlet and erased all of Thanos' actions, including Cap's ignoble death.
When Major Vance Astro was introduced, he had a special suit that allowed him to live far into the future in suspended animation. Once he got to the future and joined the Guardians of the Galaxy, he had to make sure to remain within his special suit or else he would die. The suit was originally made out of a special copper alloy, but when the Guardians got their own ongoing series in 1990, the suit was upgraded to adamantium.
Even in the future, adamantium was treated as unbreakable. However, that turned out to not be the case, as a metal stronger than adamantium was eventually found. It was called Dargonite and it was used by the gang of psychopaths that called themselves the Punishers, who fashioned bullets made out of the stuff to attack the Guardians of the Galaxy when they came to Earth. One of the bullets punctured Astro's adamantium suit. Fortunately, Astro was not only saved, but a way was found where he didn't have to stay in the suit anymore.
During the 1990s, one of the most common things in comic books was to have Wolverine guest-star in other comic books. It happened all the time. However, that was a bit of a problem with some titles, as there really wasn't any realistic way to have Wolverine guest star in, say, a comic book set a thousand years in the future like "Guardians of the Galaxy." That series got around that little problem by introducing the villainous Rancor, who was a descendant of Wolverine and actually carried with her one of Wolverine's adamantium claws!
Later, a somehow-still-around Doctor Doom put his own brain into Wolverine's adamantium skeleton, and eventually we learned how Rancor got Wolverine's claw in the first place. It turned out that there had been a war centuries earlier between mutants and the Shi'are Empire, and in a major battle during the war, Gladiator tore out one of Wolverine's claws.
7 PRELATE SCOTT SUMMERS
Charles Xavier's son, the powerful mutant known as Legion, tried to prove himself to his father by traveling back in time and killing off Magneto when he was a young man, before he became Xavier's greatest adversary. However, the younger version of Xavier sacrificed himself to save his friend Magnus' life. This major mutant attack woke Apocalypse up early and Apocalypse ended up conquering the world, creating the "Age of Apocalypse," which had altered versions of the histories of all of Marvel's characters.
Scott Summers ended up going to work for Apocalypse (via Mr. Sinister), while Logan still got his adamantium. In a battle between the two, though, Summers' optic blasts were enough to destroy Logan's adamantium-laced hand. In a striking sequence later in the "Weapon X" series, though (the name of the series that took the place of Wolverine's comic during "Age of Apocalypse"), we learned that Wolverine's claws were still in his forearm when the attack happened, so in a dramatic moment, he popped his claws out of the stump on his arm!
6 KING THOR
After one of the deaths of Odin (few characters seem to die as often as Odin), the king of Asgard gave his powers to his son, Thor. Thor was now the king of Asgard and was extremely powerful. He was so powerful, in fact, he was basically interchangeable with Odin. Asgard ended up on Earth as a floating city and soon gained thousands of new worshipers, who turned to Thor as their god. Things advanced until Thor began treating himself like a god on Earth.
Ultimately, he ended up conquering the planet. The superheroes of Earth fought against him, but that did not work out for them. He melted Wolverine's adamantium skeleton with a blast of Odinforce and then also broke Captain America's shield and killed his old friend. Eventually, after he realized how bad things had gotten, King Thor went back in time and changed history by showing his younger self the future, so that he could avoid it.
5 ULTIMATE HULK
In the Ultimate Universe, adamantium is treated a bit differently. It has a unique property that allows it to block telepathy, while at the same time it is otherwise not quite as unbreakable as it is in the regular Marvel universe. The Ultimate version of Hulk, for instance, broke an adamantium needle used on him early on in the "Ultimates" and it did not seem like it was much of a struggle.
The most famous example of the Ultimate Hulk breaking adamantium, however, came later on in the "Ultimate Wolverine vs. Hulk" miniseries by Darren Lindelof and Leinil Francis Yu, where Wolverine was sent by Nick Fury to kill the Hulk. He found the Hulk in the Himalayas, but their battle proved to be quite short, as the Hulk promptly just tore Wolverine in half and threw the two halves of his body to different sides of the mountain where Wolverine had found the Hulk.
4 ULTIMATE LONGSHOT
One of the most drastic changes between a character in their regular Marvel version to their Ultimate version was Longshot. In the regular Marvel Universe, Longshot is not a mutant, but an alien who is kindhearted and has special luck powers. In the Ultimate Universe, he still has the luck powers, but he is a mutant and not particularly nice. The X-Men met him when they helped rescue him from a reality show that he was trapped on where he essentially had to fight for his life. However, after rescuing him, the X-Men discovered that the murder he had been accused of (as an excuse to put him on the reality show) was actually legit. He really was a murderer!
Later, after getting captured, the villainous Lady Deathstrike, who had an adamantium skeleton, attacked the X-Men in a prison where Longshot was being held. He escaped and then snapped her neck, paying back the debt he felt he owed the X-Men for saving him when he was on the deadly reality show.
3 ULTIMATE VALKYRIE
In the Ultimate Universe, Valkyrie originally showed up as one of a group of ridiculous superheroes that Hank Pym joined after leaving the Ultimates. They were called the Defenders, but they were all just jokes. Valkyrie was a young woman who claimed to be a martial arts expert but barely qualified for an orange belt. Later on, though, Valkyrie was given actual superpowers. She soon became the lover of Thor and fought at his side.
After the "Ultimatum" crossover, Thor was killed in battle against Magneto. Valkyrie seemed to have died, as well, but she managed to survive and take on the hammer of Thor. She attacked the Ultimates as part of a plot to bring Thor back to life. She attacked Captain America and his adamantium shield (he "only" had an adamantium shield in the Ultimate Universe) and broke it with her magical sword. She eventually managed to bring Thor back to life, but only by trading her life for his.
2 THE SERPENT
During the crossover event known as "Fear Itself," it was revealed that before Odin was the king of Asgard, there was an earlier king: Odin's older brother, Cul. He was so evil that Odin had to not only defeat him and take over, but he also had to wipe Cul's existence from the record books. However, that was not quite enough to keep Cul from returning, and during "Fear Itself," Cul came back on Earth and slowly used his fear powers to turn the entire planet against each other. It was getting so bad that Odin considered that he might have to just destroy Earth to keep Cul from reaching Asgard.
Thor broke from his father to save Earth and ultimately managed to defeat Cul, dying in the process (but gods tend to treat dying like a minor inconvenience). Before we got to that point, though, Cul cut a swathe through the superheroes of Earth, especially once he empowered his children (known as the Worthy) to possess a number of powerful heroes and villains. In one battle, the Serpent just destroyed Captain America's shield like it was nothing (later, it was rebuilt using Asgardian metal).
As noted, The Serpent was aided by The Worthy, who were his children that he transformed into magical hammers, a la Mjolnir. When Cul was freed, so, too, were they. They shot across the globe, where each hammer was found by a different powerful Marvel character. When the villains and heroes picked up the hammers, they were transformed into the children of Cul, with their initial power set being amplified by the power of the Worthy. Hulk picked up the hammer of Nul, the World-Breaker.
Nul in possession of Hulk's body was stronger than even the Hulk. Nul took on Dracula in a mighty battle, during which the Lord of the vampires entangled Nul in an special net made of adamantium. Nul shocked Dracula by tearing right through the net. Eventually, Nul was stopped by getting the Hulk to break free of Nul's control (Nul became one of the only members of the Worthy to last past the "Fear Itself" event).
Can you think of any other time adamantium was broken? Let us know in the comments!