Getting rid of all this nuclear waste is going to be expensive. Eh, it's cheaper to just slap some concrete over it. It'll be fine!
During the long construction of the Nine Circles, the subsurface artificial gravity generators were among the most important assets to bring online. These devices consume a tremendous amount of power, and to kickstart the process a number of cheap fission reactors were installed until the much larger fusion plants were installed.
The fission plants were subsequently decommissioned and dismantled. However, there was a problem for the firms contracted to decommission the old reactors. Disposing of hazardous nuclear waste is extremely expensive, so one firm decided it simply couldn't be bothered. They simply forged the nuclear waste vitrification paperwork and the expense forms, slapped concrete over everything, and quietly pocketed the profits. These concrete tombs became known as Vitrification Bunkers, and each was assigned a colour codename to distinguish between them.
The Indigo bunker located on Tertius
is an example of a particularly shoddy job. The concrete is crumbling and groundwater is seeping in and out of large cracks. Most of the population of Tertius are blissfully unaware of the hazardous materials contained inside, and thanks to the forged paperwork, not even the company responsible realises that the nuclear waste hasn't been properly vitrified and is still water soluble...