Biogeocomputers utilise a deep hot biosphere layer deep in the crust
In the first century AT it was discovered that the crust of the Earth, down to a depth of up to ten kilometres, was populated by an ecosystem of chemosynthetic bacteria living on hydrogen generated from igneous crustal rocks. It was estimated that the total mass of this crustal ecosystem could, perhaps, be as great as that of the entire land and sea ecosystems of the planet. It was theorised that the conditions allowing such as ecosystem to exist would be found in any Earth-like world in or outside the life zone.
Not long after this discovery, it was speculated that if the native crustal bacteria could be replaced by genetically engineered bacteriological computers communicating chemically, or perhaps by chemically-powered nanomachines, then the entire crust of a planet could be turned into a vast, self-powered and very physically secure, if rather slow biological computer. This was dubbed BioGeoComputing (BGC).
When it was discovered in 2136 AD that, as theorised, Mars possessed a crustal environment suitable for BGC, but empty of life, the Mars Development Corporation (MDC) began experiments towards the creation of a Martian BGC system. However, various technical problems dogged the project, and with the coming of the nanite swarms, the project stopped, and the BGC concept languished in obscurity.
As humanity recovered and began to spread out into space once more, several groups experimented with the BGC concept, but technical problems prevented the successful application of the principal to more than a few cubic kilometres of crustal rock, which, although cheap and easy, could not compete with more conventional computer systems. When life-bearing extra-terrestrial worlds were discovered, almost all were found to have thriving crustal ecosystems, much like that of the Earth.
The Zoeific Biopolity were the first group to fully develop BGC technology, and in 4166 introduced BGC as part of the terraforming process of the world Kdregra in the Vulpecula region. Since then the Zoeific Biopolity have introduced BGC to many of their worlds, usually as part of the terraforming process, but in same cases later in a world's development. The BGC systems are used provide vast and ubiquitous computer systems to new settlements (by their drilling down to the 'computing strata'), and for large-scale computing efforts, such as ecosystem modelling. Many also run significant virch worlds.
Because of its cheapness and ease of installation, BGC has spread from the Zoeific Biopolity far across Terragen space, becoming widespread in the core worlds of the Biovirate (before its destruction) and a standard part of the planetary terraforming process in places far from the Zoeific Biopolity. BGC installation was also incorporated into some versions of the Yggdrasil Bush. Of course, this means that there are now many variant BGC ecosystems, developed from the original by various groups.
In general the BGC ecosystem will share a common genetic descent with any surface ecosystem; this is certainly the case with BGCs created by Yggdrasil Bushes. However, for extra security and isolation some worlds use BGCs that are engineered to be very different to the surface ecosystem.
In all cases it is much easier to introduce a BGC into a lifeless planetary crust rather than use it to displace an existing crustal ecosystem. Whether life exists both on the surface and in the planetary crust, or only in one of these two environments seems to depend very much on the evolutionary path a given world has taken.
In some places virch entities have used BGC to provide themselves with massive physically secure computing resources with no associated surface ecosystem. It is speculated that some Hider groups may have used similar techniques and migrated into various hidden BGC ecosystems.
Some polities have successfully extended the BGC concept from biological to nanotechnological computing elements, communicating via physical rather than chemical means and drawing additional power from geothermal power tap threads. Although more expensive and somewhat harder to create than BGC systems, these NanoGeoComputing (NGC) systems have also spread quite far across Terragens space.
The first use of BGC in war was made during the Biovirate-Keter War. The Biovirate managed to secretly seed the crusts of a number of Keterist worlds close to their borders with hostile BGCs - what have since become known as geowars. When these geowars matured, they began infiltrating bioweapons - both poisons and hostile living entities - up out of the crust to disrupt the world above. The Keterist worlds of Glapstone, Conwald and Karadan were devastated by these seemingly unstoppable attacks from below, and the infected worlds in other systems badly damaged.
At first nothing could be done to stop the geowar attacks other than quarantine - the core of the geowar systems were sufficiently far down to be essentially untouchable by conventional means. However, in time, and once the war was over, the Keterists developed anti-geowar systems with which they cleansed the infected worlds of the Biovirate geowar systems, restoring them to habitability.
Since then, even in systems that do not use BGC, the crusts of worlds that consider themselves at risk have been infected with anti-BGC micro-organisms. Many of those worlds that do have BGC have also taken increased precautions to prevent the infection of subversion of their BGC systems. And despite this, occasionally a geowar is used in war somewhere in Terragens space, to the extent that several worlds have been rendered uninhabitable by them.
In 7852 the first alien BGC system was discovered, on Steelcliff, an isolated world towards galactic anti-spinward in what is now the Negentropy Alliance Outer Volumes. It appears to be running some kind of virch world, and certainly has sufficient computing resources to run at least millions of entities. It seems to have been put into place some forty million years ago, but as yet no attempt has been made to contact who or whatever may exist inside it.
With the discovery of the planet Whisper in the same area of space in the early 9000's, it was found that the lifeforms of Whisper and the BGC bacteriocomputers shared a good deal of genetic material, anti-mutation protection and biocomputing methods, certainly far too much to be a coincidence. However, it is currently unclear whether the much younger Steelcliff BGC was put in place by the same race as the creators of the Whisper ecosystem, or whether another race derived the BGC ecosystem from that on Whisper.
Since then, other alien BGC ecosystems have been discovered on a number of worlds further to anti-spinward. At first thought to be more examples of the Steelcliff BGC system, analysis proved that they were in fact completely unrelated to it, but very similar to one another. In one case the BGC system was also very young, not more than a few hundred years old and still spreading through the planetary crust.
The fact that this area of space is the part of Terragens space closest to the Low Emission Zone has lead to the theory among some Terragens xenologists that perhaps there actually is a civilisation in the Zone, but that it uses BGC to colonise worlds using artificial panspermia and is either highly solipsist or is hiding for some reason. Several more paranoid speculators hypothesise that the apparent absence of any other civilisations in the Zone is due to their destruction by the alien BGC civilisation and that the difficulties detected in HEEC 1 are a result of subversion by the BGC civilisation as it continues to expand around the galaxy. The fact that all but one of the worlds discovered to contain alien BGC systems share an origin indicates to these theorisers that the Low Emission Zone civilisation is now starting to appear in Terragens space, and they are now calling for Something To Be Done, despite the lack of any solid evidence one way or another.