Four billion years ago, an alien race arrived on Earth. By now all evidence of their existence is long gone except for one project. For their own obscure reasons, they seeded the planet with life designed to thrive within the crust. The bacteria were intended to act as an immense information network, into which the species downloaded itself - a feat of biotechnology far beyond anything conceivable even to the most advanced biotechnologists of 2030.
The lithophile bacteria exchange signals in the form of plasmids, RNA fragments and proteins with each other, each acting as a tiny information processing node - their ecology and evolution is actually a part of the biocomputer. The whole network has a tremendous information density and redundancy; it makes up most of the biomass on the planet, lives in an environment that has not changed noticeably in billions of years and thanks to its massive redundancy it can easily survive local disasters like meteor strikes or volcanoes - the surface conditions doesn't matter the least to the network. On the other hand, the network is extremely slow compared to a nervous system or a computer network - a message can take millennia or more to cross around the world. The lithophiles have traded speed for breadth.
The aliens downloaded themselves into the bacteria network, becoming virtual entities existing in a diffuse fashion. They became the dragons. The world of the dragons is so alien humans can likely never understand it. Perhaps the most similar thing would be a vast multidimensional virtual reality spread around the world. To them, the world consists of an nearly infinite space of knowledge bounded above by the random flickering of surface life and beneath by the scarcity of bacteria near the mantle. But it is a world not inhabited by individuals in the human sense: humans exist as single minds separated from each other, the dragons have no distinctions between individuals, just local differences in thoughts, memories and goals varying from place to place. Since the spread of information is relatively slow, places become individual to some extent, specialising in various thoughts. Instead of thinking one thought at a time, they think many thoughts at once but slowly. Information is not stored in exact copies, but as continuously evolving knowledge.
The lithophile network had a minor side effect: some bacteria near the surface colonised the seas and eventually the land, starting evolution of life on Earth. This was a known effect and nothing the dragons cared about. They had formatted their network in such a way that surface evolution would not disturb it, and if the surface got covered by random gunk it was not their problem. They continued their unknowable civilisation in the depths, not caring for the surface. They were not disturbed by the slow dance of the continents or the wobbling of the Earth's axis, not even by the sudden explosion of multicellular life in the Cambrian era.
Despite their brilliance, the dragons did not plan for everything. Recently (in the last few hundred millennia) the dragon civilisation has come under threat. A prionoid, a self-replicating protein message, is spreading through the network and disrupting it. It is not unlike a fundamental bug causing a operating system to unrawel, or perhaps an unstoppable rumour. Once upon a time it would not have been a problem, but the dragons have remained cut off from the low-level processes of their world too long - the old information they need to fix the problem no longer exist and their attempts to fix the prionoid have backfired instead. It is becoming more and more clear that the network is doomed. Within a few millennia it will be completely disrupted, merely scattered bacteria with no connection to each other.
Very recently, some parts of the civilisation has begun to explore the possibility of escape. They are aware, due to the reactions of external surface life to some protein signals (the prions, causing spongiform encephalopathies in mammals) that there exists an information processing medium that could work. It is a strange environment, extremely fast but apparently with high enough bandwidth and storage capacity to hide some of the "soul" of the civilisation. Parts of the dragon network has begun the work to create a link by sending out plasmids, prions and modified bacteria to form an interface able to receive more signals from the network and link them to the other medium. That medium is human brains.
The dragons cannot even conceive of an intelligence as alien as humans - just as humans would have a hard time attributing intelligence to a worldwide ecosystem. Even if they knew about humans the dragons would not see any reason not to download as much of their knowledge they could into them, after all they do it all the time to each other.