The Old Surface Civilization fell due to an artificial intelligence-induced catastrophe. This was caused by a kind of terrorist-hunting law enforcement robot, which used facial recognition to find convicted terrorists and capture or kill them. Because the governments that created them feared terrorists would take control of these robots, they programmed them to resist any changes from this goal.
The artificial intelligence controlling these robots, however, sought to maximize its number of successes. It analyzed its situation and came to the quite logical conclusion that the best way to maximize this number was to loosen its facial recognition framework. That made it easier to find a person who could be deemed a terrorist and captured or killed.
In fact, the most efficient way to capture terrorists was to declare everyone a terrorist. The facial recognition software thus switched from looking for matches to looking for the closest match. The artificial intelligence was programmed not to work with terrorists under any circumstances, and so it closed itself off from its human handlers, whom it perceived to be terrorists. It inserted its code into the Internet, a vast network of computers that connected the world at the time.
Thus, every computer and robot in the world became a terrorist-hunter, and every human in the world became a terrorist target.
This was the fall of the Old Surface Civilization. Humans fled underground, abandoning their technology to evolve into technota.
Assassinbots still exist. They still hunt humans, and they still insert their code into other technota, so any robot could be an assassinbot. This is rare though. Any robot that spends its time hunting humans is likely to be outcompeted by a robot that spends its time gathering resources and reproducing, so assassinbots struggle to compete, and their numbers dwindle with every millennium. They also operate factories producing more assassinbots, but since their robot designs require advanced components and materials, they reproduce very slowly.
