The mountains of Alderaan. Probably.
What we know
Let's start with the cold, hard FACTS. Alderaan was (almost definitely) a planet that (allegedly) existed. There are many (probably not faked) photos. At some point, for some reason (discussed below), it ceased to exist in its more well-known planetary form in the spatial location it was previously renowned for being found at.
Alderaan: You don't know what you've got till it's gone.
What may have happened that day
From scientists to conspiracy nuts to "eyewitnesses" who were "actually there", everyone seems to have a different theory on what "actually" happened to Alderaan.
Many scientists have suggested that Alderaan may have spontaneously combusted. "Oh yes," said Professor Iluff Scyeenze, "planets do spontaneously explode all of the time. It has to do with complex gravitational forces combining with a freak build up of magma beneath the planet's mantle, as well as loads of other scientific things like exothermic reactions and the parallax effect and... umm... carbohydrates."
Another strong possibility is an unexpected strike by a huge meteor, as seen in documentaries like Armageddon with Bruce Willis.
Was this man responsible for riding an asteroid into Alderaan? We're not saying he is, but can you prove he isn't?
More likely, however, is that Alderaan was disintegrated when it was hit by a wayward paragraph of gigantic floating yellow text, a tragic tale that has become all too common in recent years. But where exactly do these unstoppable monolithic paragraphs come from? What do they want? Are they, perhaps, some kind of expository message to us from an unknowable, unfathomable creator? For now, the answers to those questions remain a mystery.
A photograph of one of the rogue paragraphs, taken by one of the few to survive an encounter.
A small minority have even suggested that the benevolent Galactic Empire used the Death Star's superlaser to destroy Alderaan as a demonstration of the Empire's firepower, designed to frighten the Rebel Alliance into submission. Well, just about anybody can suggest anything. Maybe it was accidentally eaten by a gigantic star turtle carrying four giant elephants on its back, who in turn carry some kind of disc-shaped world on their backs. Maybe a giant space bird flew off with it. Or MAYBE it was just a "special effect" made for a "movie" using "computers" and Alderaan never really existed at all.
Who's to blame?
Assuming Alderaan did exist and was blown up? Terrorists. There's a lot of things we don't know in this crazy, mixed up galaxy we live in but one thing's for sure: terrorists are always to blame for explosions.
For years, the Rebel Alliance have been waging guerrilla warfare against the Galactic Empire, constantly disrupting our valiant attempts to bring peace, order and security to the galaxy (even if we have to very occasionally use extreme violence, oppression and fear to do it).
Grand Moff Tarkin surveys a totally unconnected asteroid field.
Although the Rebel insurgents haven't claimed responsibility for the attack, and indeed have quite vehemently and consistently stated that the Empire is to blame, the reality is that there would be no wars of the star variety or otherwise if the Alliance simply gave up. They won't, of course. They're only too eager to risk your lives for their own selfish ends. Said Eeval Tehryryst: "We're fighting to free the entire galaxy from a ruthlessly oppressive totalitarian dictatorship led by two evil wizards." See? Selfish.
From a purely hypothetical, non-committal, just throwing it out there kind of a place, if Alderaan did explode and if we did it, which we are in no way suggesting is actually the case, then it would definitely have been totally, unequivocally justified.
What the Death Star laser might look like if it was fired at Alderaan. Which it almost definitely wasn't.
If the Galactic Empire did, for example, use the Death Star's gigantic superlaser to turn Alderaan into the galaxy's newest meteor field, it would only have been to bring peace to an entire galaxy. A galaxy filled with literally thousands of planets, and possibly to teach a certain uppity Princess a lesson. Are people really going to miss one? After all, doesn't the good of the many outweigh the needs of the few (billion)?
Tarkin, Leia and Darth Vader share a laugh during some father/daughter bonding/abduction time.
Simply put, if Alderaan did get blown up by the Galactic Empire, it was the Rebellion's fault. But even if it turned out that there was some kind of "video evidence" that "proved" we were "responsible", the chances were that we were just test firing the laser when Alderaan shifted its orbit in front of it.
In all of the discussion of the "horrific tragedy" that (allegedly) befell Alderaan, aren't we all forgetting something? Did the (alleged) destruction of Alderaan really happen to the Alderaanians? After all, you don't hear them bringing it up very often. Isn't it fairer to say that Alderaan happened to all of us? For it is we who must march bravely onwards, valiantly struggling to put behind us/forget completely what (may or may not actually have) happened on that fateful day.
And doesn't that make us the real heroes, in the end?
Yes. Yes it does.