Zero Dark
The Zero Dark was the forty or so years following the Comet Strike when the earth was in a state of crisis consequent to the strike and its effects. No humans witnessed the Zero Dark, and the only surviving records were made by machines.
The first decade
The Comet fragment seems to have been quite small, maybe 20 metres or so across judging by footage recorded by the space hotels, but it hit in the very sensitive Yellowstone Caldera, or close to it, causing the supervolcano complex to reactivate. The shockwaves travelled round the earth and met on the other side of the planet causing a reactivation of the Kerguelen hotspot just north of Antarctica. The submerged Kerguelen minicontinent once again began land-building. All the remaining ice of Antarctica left over from the Helios Fail melted or evaporated.
Most of the hanyo bunkers under the Earth were shallow affairs not designed to be occupied indefinitely. They had air-scrubbers and minimal waste recycling, but food was mostly stockpiled and the plan was for the hanyo corporate bosses to return to the surface in 2130 when the Robot Wars should have ended. Hanyo bunkers that were not destroyed in the impact were rendered unviable by atmospheric conditions. The space hotels were also not self sufficient and needed regular restocking from Earth. This was now not an option, and the space hotels rapidly plunged into anarchy, war and cannibalism.
The survivariums, on the other hand, were built very deep in the rock and suffered little damage from the impact, except for any that may have been in North America or Japan. The isthmus of Panama sank and allowed the Pacific and Atlantic oceans to communicate, while Tierra del fuego and Antarctica acquired a landbridge. Japan broke apart and sank. Kilometre-high waves of mud and water remodelled coastlines. and there were simultaneous volcanic outbursts on every continent. This was followed by firestorms as the remaining vegetation on Earth burned up. As this phase settled down, the acid rains and overcast skies set in, rapidly cooling the Earth and forming black ice in the higher latitudes. Slowly, as the blizzards relentlessly piled up snow everywhere but the Equator (which was later to form part of the Green Zone), the ice turned gray, then white. Ice formed much more rapidly in the north, covering Europe in a kilometre-thick glacier sheet. The south remained lightly frozen, probably because of hot water circulating from the Kerguelen hotspot. A new Ice Age began.
The second decade
Both the Yellowstone supercaldera and the Kerguelen orogeny, being close to polar regions and subject to high winds, blizzards and ice formation, created areas of extremely high winds and storms. As Kerguelen remained mostly underwater till about three centuries post the event, it did not contribute significantly the the ash content of the atmosphere. It did, however, heat the southern Indian Ocean and create an anoxic zone. Solar radiation continued to fail to penetrate the cloud cover.
Around this time the machines still surviving on the surface of the earth began to come out of hiding and videograph the devastation. The more violent effects had now settled down and the hazards were mostly from the acid rain and the almost constant earthquakes. The machines did not require solar radiation to survive as they could at a pinch create food from crushed limestone and algae. However, exploration was difficult because of the vast quantities of steaming unstable mud thrown up by the tidal waves.
The third decade
Cloud cover began to lighten slowly, although the skies were still very far from clear. The lava sheet over north America had crusted over and was delivering less heat to the atmosphere. The Eden space hotel belonging to Hasgod corporation lost orbital stability and re-entered Earth's atmosphere, breaking up over the Pacific Ocean. The machines had reestablished communication with each other and now began to take stock of the ruin. They drew up possible places to search for human survivors.
Africa and Asia remained the least affected continents and the machines decided to concentrate their efforts there. As they searched, they also collected human artefacts, took samples of plants and seeds and rescued any living creatures they found. They soon acquired a number of rats and a copy of the Karma Sutra. They created depots in safe places for their finds and also deposited backups of their videologs for storage.
The fourth decade
At first the machines had little success. The equatorial zone appeared to be stable but deserted. After a futile search of the Persian desert they began to venture northeast, where they found to their surprise that the Tarim Basin had become a small inland sea. They skirted this sea and began moving northwards. From their reading of the Karma Sutra they reasoned they would most likely find survivariums in the mountains. When they came near the Flaming Mountains they began to detect a faint, fugitive signal. They triangulated it as best they could and discovered the underground listen post of Zigsa's Nest. It was 2174.
