This Tinfoil Hat post is not meant to be taken as established lore, but merely as an exploration of what that lore could mean.
Sometimes an idea starts small. I was musing about Saronite, the literal blood of an Old God, which as we all know was used as the material that created Icecrown Citadel. Then I thought about how the last dying breath of an Old God became the Sha, actual embodiments of corruption.... and how, even after the death of that Old God, the Heart of Y'Shaarj could taint the entire Vale of Eternal Blossoms. The very substance of an Old God... its blood, its flesh, even its last breath can taint, warp and corrupt the world.
Then I started thinking about the madness the Old Gods engender. Upon first arriving in the Howling Fjord, members of the Explorer's League were driven mad by the thoughts of Yogg-Saron, trapped within the Whisper Gulch. Yogg-Saron, after all, was massive - his tendrils extend all the way across the continent, from Icecown through the Storm Peaks and down into the Dragonblight, the Grizzly Hills, into the Fjord itself. And this got me thinking something else.
Northrend is dominated by Yogg-Saron... but the Old Gods predate the Sundering, and so when all continents were part of the great original Kalimdor, that means that the Old Gods lay submerged beneath it as well. The Old God N'Zoth most likely lay beneath the center of the primordial landmass, ancient C'thun lay to its west, and before its death at the hands of Master Ra and the Mogu the south was the domain of Y'Shaarj whose seven heads consumed hope and begat despair. But many were the Old Gods, and powerful (or so the Klaxxi maintain) and this leaves me to ask - was there a god to the East? And what became of it?
The Destroyer destroys Kezan
Many strange things happened during the Cataclysm - as Deathwing rampaged against the world, some places he sought out and directly attacked. One of these places was Kezan, home to the goblins. Goblin history begins with the goblins as drudges, slaves of the mighty troll empire that once ruled much of Azeroth entire, forced to mine deep in the earth for their troll masters. Until they found kaja'mite, that is. Kaja'mite changed goblins, making them seem far more intelligent and allowing them to free themselves from their trollish overlords. Indeed, the situation on Kezan changed so much that goblins actually had troll slaves there. The kaja'mite became essentially the heart of goblin industry and society, as its influence allowed the goblins to rival the inventiveness of the Titan created gnomes, and Kezan became the center of a society built around innovation ... ruthless, heedless, without regards to morality or honor, certainly, and innovation based not just in the invention of technology, but in matters of trade as well. Goblins under the influence of kaja'mite became masters of the art of the deal, and dealing with a goblin was always something others were wary of. One counted one's fingers after clasping hands with goblins.
This mercantile society, equal parts greed and eureka, thrived until recently. During the Cataclysm, while the goblins were finding new deposits of kaja'mite on Kezan (the ore had become so plentiful that the goblins had taken to crushing it and drinking it in a beverage called Kaja'Cola) the destroyed arrived, and much of the surface of Kezan was buried by the volcanic eruption of Mount Kajaro. Those that managed to flee ended up as the Bilgewater Cartel that serves the modern Horde.
It seems obvious to me that Deathwing buried Kezan on purpose, and since Deathwing has often made use of goblin engineers (and didn't go out of his way to kill the goblins fleeing Kezan) it wasn't that he cared much about the goblins one way or another. No, there was another reason to bury Kezan, and that reason could only be the kaja'mite. The element that made the goblins seem so smart, that gave them ideas about trade, about the creation of technology. The kaja'mite freed the goblins from slavery... so that they could enslave their enslavers. The kaja'mite improved the goblins ... but it didn't make them kinder, or more caring, it didn't provide a sense of community or concern for one's fellows. It either had no moral effects at all, or it had negative ones - keep in mind that Trade Prince Gallywix attempted to place his own people in slavery after extorting every last bit of profit he could in offering a means to escape the exploding island. Goblin society does not care about ephemeral concepts like decency or honor, it just cares about the ruthless acquisition of temporal power via any means necessary. Goblins are smarter, yes, but they're just as brutal.
So there's enough reason to destroy Kezan - can you imagine gnomes on Kaja'Cola? Perhaps there's an even more compelling reason to bury the stuff, however. Because we haven't yet determined what kaja'mite is, but we know this much - it comes from deep underground, and it affects the mind. Remember Whisper Gulch? Where the blood of Yogg-Saron, the ore saronite, was wrenched out of the deep earth and drove the miners of the Explorer's League mad... specifically, it drove them mad because they could now see the truth. Now, let's look at kaja'mite again. In its raw form, it seems to generally cause an increase in intelligence - when the 'juice is distilled' from the kaja'mite, the resulting beverage seems to act to immediately cause the imbiber to have an idea. It's literally inspiration in a drink.
How you may think, why you may think it
This begs several questions. First off, since when does a mineral have juice that you can distill from it? Secondly, while some of the ideas the Kaja'Cola induces seem, well, impractical, some seem fairly sound - in fact, it often seems as though the goblins themselves are the ones ruining the ideas with their additions. So let us consider - kaja'mite seems to be pure ideas, pure information. It is how without any concern as to why or even should. Exposure to kaja'mite creates intelligence, increases inspiration - you're smarter because you have more ideas. You have more ideas because more information is imparted to you. Where do these ideas come from? One possibility seems obvious enough - the resemblance between kaja'mite and saronite would make it seem likely that kaja'mite is another part of an Old God, the mysterious Old God who once lived beneath what is now the Malestrom, with tendrils perhaps extending as far as the Lost Isles, Kezan, perhaps even Stranglethorn Vale. Places goblins congregate. One remembers the mysterious artifact found in the Mosh'ogg Bounty chest, which was said to be similar to but different from Saronite.
This leads us to consider the role of Neptulon in the Cataclysm and why he was so opposed to working with the forces of the Old Gods, the naga and faceless ones - as an elemental lord, Neptulon was once one of the servitors of the Old Gods. Why did he oppose Deathwing? Perhaps because the Old Gods are not a monolith, not a unified agency - in the time before the return of the Titans, the Old Gods amused themselves by pitting their elementals against one another in unceasing war. In the past, Neptulon himself unleashed mighty krakens against the Gurubashi, sinking their cities and helping begin the destruction of their empire. This is often speculated to have been due to the presence of the Stone of the Tides, an artifact that allowed the Gurubashi to threaten Neptulon's power over water elementals, and that may have been so. But ask yourself this - how did the trolls learn how to make it? Rituals of elemental summoning and binding on that scale, so powerful that even Neptulon the Tidehunter himself thought they were a threat?
The artifact inside the Mosh'ogg Bounty reacts violently to native troll magics, and it is believed that the same blue stone was used in the Jungle Remedy that caused Colonel Kurzen to go mad, and his followers to begin using magics very similar to those used by the Zandalari on the Isle of Thunder. We know that the Gurubashi would learn many secrets the Zandalar didn't want them to have, such as the ritual to summon the Blood God Hakkar to our world. In time, the Temple of Atal'Hakkar would be smashed into a sunken ruin in the Swamp of Sorrows by the power of Ysera, the Aspect of Dreams. Aside from the evil of Hakkar's priests, why would Ysera take action against them? Well, ask yourself this question - what Old God seems most associated with the sea, and with Deathwing?
Under the Undersea
Several times while adventuring in Vashj'ir, references are made to a greater evil presence 'below' the underwater regions. The Puzzle Box of Yogg-Saron and the Heart of Y'Shaarj both make reference to this place, the sunken city of Ny'alotha. Imagine, then, that Ny'alotha and N'Zoth have a presence both in this world, and in the Emerald Dream - that Ny'alotha is the Emerald Nightmare bounded by the Rift of Aln, manifested by the presence of N'Zoth. Consider this - where Yogg-Saron lies, there lies his manifested blood, the raw saronite. Where Y'Shaarj last breathed the Sha infest the land, given terrible power by his undying Heart, the stuff of his body. Now, as for N'Zoth - his city of Ny'alotha is said to be a place of dreams, and when we walk there, we're told, we will see.
What if kaja'mite is the physical stuff of N'Zoth, effectively the Old God's brain? What if the goblins are drinking the Old God's thoughts, and what if this sleeping Old God's dreams were the ones that the trolls used to bring Hakkar to Azeroth, to threaten Neptulon's power over the elementals? The tendrils of the Old God could easily reach from the Maelstrom to Stranglethorn, especially since at once time there was merely one continent. And if that's the case, then Deathwing would have wanted the kaja'mite buried because it would contain everything the Old God knows, all its dread secrets and thus, all of the plans that Deathwing was compelled to execute. When the Maws of Shu'ma and Gor'ath rose at Wyrmrest Temple, the servants of N'Zoth were there, supporting the Aspect of Death in his rampage, and Warlord Zon'ozz is specifically mentioned as having waged unceasing war against the forces of C'Thun and Yogg-Saron - clearly there's no reason to believe that the Old Gods are very good at working together.
In the end, we're left with more questions than answers. Did the goblins force their culture upwards by literally consuming the dreams of an Old God? Did those dreams drag Hakkar to our world, brought by the secrets stolen from the calcified dreams of a being that sprawls under the seas? Probably not, but it's interesting to consider. Perhaps it's all part of N'Zoth's plan ... to let Ny'alotha spread, bringing us information without the moral guidance to properly use it, a dangerous enlightenment. To put nuclear weapons into the arms of metaphorical cavemen and chortle as we destroy ourselves, while it dreams.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.