The Unstable Mine was actually stable. It was a secret of a player who dropped out early in the game, and no one ever investigated it.
Obficina was abandoned because as the capacity for industry decreased rapidly in the years following going Underground there was no longer a need for a second Foundry.
Visio suffocated. When Dane Lawrence realised that the facility could be used to make telecommunications equipment that could, in principle, contact the surface, he decided it best to destroy the skillset. Those scientists and engineers who worked in optics and microfabrication were reassigned to the most menial role that Dane could imagine: logistics. As time went by the machinery fell into disrepair and, now distracted by the increasingly difficult work of keeping a dilapidated Underground functioning, the early logisticians never had a chance to teach another generation how to use Visio.
Had anyone walked into Sagitta, they would have discovered Dane Lawrence’s thirty foot tall intercontinental ballistic missile and a broken centrifuge in which he had been attempting to refine Uranium. He at one point had ambitions to nuke the bastards at Principium HQ but became distracted, mostly because the project was proceeding too slowly i.e. not at all because the engineers had realised what he was doing and slowly found themselves other projects to work on. Without maintenance and still running one day the centrifuge simply exploded, ejecting a cloud of radioactive particles into Sagitta and Requiem. At this point however Dane no longer had the capacity to do anything sensible about it and the Secretary, Mai, was mostly interested in what the Underground would choose to do without direction. As it turned out, leave it be.
Vox was, in fact, never broken. It certainly appeared that way from the Underground, but in reality it had simply been disconnected from the Overground array. This was, of course, done by none other than Dane Lawrence. After Heather studied it enough to know which wires to connect, fixing it from the Overground at the base of the telecommunications mast was relatively easy.
It may at this point surprise you to learn that the fire which destroyed Nexum was not set by Dane. No, rather it was set to stop him. As his experiments became increasingly unhinged towards the end of his life, in particular his attempt to engineer human-rat hybrids that would be ideal for Underground life, the workers at Nexum took it upon themselves to destroy their lab and their research.