The planet Bau is home to a post-industrial civilization that's spaceflight program was cut short in a most illustrative way for anthropologists.
It is unclear when the Bauistas' ancestors landed on Bau, but the best data available suggests that it was approximately 20,000 years ago. Bau's relatively proximity to the Blowhole suggests that their ancestors traveled through the Bottlenose Nebula itself to reach the world, but as no clear genetic lineage links them to known world in the Nebula, it is presumed that their original homeworld has either been destroyed, or they originated from very far away, indeed.
By at least 15,000 years ago, it appears any indicators in their records that they were not indigenous to the planet had been erased. They were under study by a number of civilizations, most pivotally the Rothans, once they achieved the industrial age and began to adjust their planet in ways that were detectable at interstellar distances.
Just two years after the Bauistas made their first forays into orbital space, though, the Happy Hat Incident brought the Rothan Cloud crashing down from the stars, and extraterrestrial robots sacked every computer, museum, home, business, building, and den on Bau in search of Happy Hat vectors. The Rothan Cloud obviously found none, but the coming of the bots, their seven-day raid of the world, and their abrupt disappearance traumatized Bauistan society in a way few if any other societies have ever suffered.
The people of Bau (demonym: bauistas) are a carbon-based lifeform with virtually no genetic information or compatibility with the life on their planet. There is strong evidence that the civilization is actually a lost colony from some other, likely now-extinct interstellar-capable people. As the Bauistan people uncovered advanced microbiology and other industrial-age scientific discoveries, they arced strongly in the direction of believing they are fundamentally different from all other live in the universe. This conceit is strongly supported by all biological evidence they have access to. As a result, the prevailing scientific view was that they were creating by a higher power, granted dominion over the world of Bau, although the nature of this higher power (supernatural or merely extraterrestrial) was the subject of intense debate and a small number of armed conflicts.
A second unexceptional, but as it turned out, critical belief among the vast majority of Bauistas, was the concept of Soleidan, roughly translated as "a life completed" or "a life well-lived." It was a central tenant that once an individual had either succeeded or failed at their life's main goal, that there was no requirement that their life continue. In some societies, this simply made self-euthanasia legal after a certain age or career milestone, while more extreme Bauistan societies actually would involuntarily execute members of their society who met certain criteria. Achieving Soleidan, voluntarily or otherwise, was considered anything from an honor to an expectation, but violence or murder committed against an innocent Bauistan who had not yet qualified for Soleidan was universally a more serious crime than against one who had, as it was seen as squandering the potential of another.
The Happy Hat Incident
These beliefs ended up interacting in informative ways in the aftermath of the Happy Hat Incident. In the first few hours of the landings, the prevailing narrative among the Bauistas was that the Creators had returned, either to cast judgement upon the people of Bau, or to usher them to their new destiny among the stars. As the bots began to smash cultural institutions (although fastidiously not harming anybody who didn't attempt to stop them), the judgement camp began to take control of the narrative in the media of the time. Surely, the leaders said, the visitors were here to determine if the Bauistan people had achieved a spiritual level sufficient to proceed to the next stage of their evolution, or if they would be destroyed to be replaced by another, more worthy race. Both were given the mantle of a kind of species-level Soleidan, that either way the story of the Bauistan society was drawing to a close. Significantly, the Rothan bots discovered the previously-unknown Bauistan copy of the Kandar Lithoglyph, bringing the number of known copies to six.
When the Rothan bots left after seven days of searching, the world held it breath for another week, awaiting its judgement.
After two weeks, nothing came. After a month, the economic backlash of an entire workforce believing tomorrow wasn't coming caught up with the realities of life on Bau. As reserves of key spare parts and other materials started to run dry, the society started to snap out of their 3-billion-strong stupor. Fortunately, the Bauistas were able to recover before critical needs like food and medicine completely ran out, but as the industrial base of the civilization staggered back to life, major questions came along with it. Had they been judged? Were they still being judged? Did anything they did now, after the visitation, affect the future of their society as a whole? Had they simply been left to die?
There was not widespread conflict, as one might expect, during this period of uncertainty. A small cadre of Bauistas were able to reframe the enormous narrative that had built up around the visitation by creating a duality; the Bauistas had gone to Space, and the people of Space had judged them unworthy of attention. Space was closed to them, and rather than up, they should look down. All the energy, resources, and drive that most civilizations put into space travel were, in the space of a few months, redirected to the Bauistans tunneling into their own planets, and within a generation they had become an almost entirely subterranean civilization.
Today, the Bauistas still have some farming and energy collection apparatus on the surface of their work, but unless one has to venture to the surface for their work, no Bauista ever goes there. When this shift was first discovered, the assumption by off-world anthropologists was that these bunkers were built to protect the population from a second invasion, and hence were called Happy Hat Protection Vaults. The name has stuck, even after the true purpose of the underground works was discovered.
As a result of their extraordinary species-level pivot, the Bau's understanding of planetary geology, as least as far as continental worlds go, is among the most advanced in the galaxy, and certainly the greatest among single-planet, subluminal civilizations. There is, in fact, a small but significiant industry in superluminal geology scholars sending agents to Bau to obtain their data and writings about their own planetary geology, as combined it represents the most comprehensive close-in examination of a single planet to date.