The Strange Worlds' tunnels are self-maintaining to a remarkable degree. Light, heat, and breathable oxygen/nitrogen air are all provided. Gravity and air pressure are identical to sea level on Earth. This couldn't be a coincidence and many researchers have concluded that the tunnels must have been made by humans, or at least by somebody from one or another Earthly reality.
But there are also some intriguing services that are not provided. Water and food are not available. Free water evaporates at a rate much faster than one would expect given the humidity. Seeds do not germinate and plants wither in the otherwise healthy artificial light (there are no nights in the tunnels). There is no wind, and no animals seem to routinely live in the tunnels.
And there is no trash pickup. Consider the implications -- at any given time, the twenty or so miles closest to Nexus City are traversed by thousands of sentients. There are no bathrooms, no trash cans, and no scavenging vermin.
Instead, there are the Sweepers. These elite, well-paid specialists are based in Nexus City and periodically ride down the tunnels in their familiar low slung go-zambonis. Dirt, grime, and casual detritus are vacuumed up and periodically discarded into designated "junk-yard" realities. Problematic trash -- bodies, particularly slow moving (or sleeping) sentients, or abandoned property -- is carefully evaluated by the experienced sweepers and disposed of as appropriate. Construction, decorations, or (as occasionally happens) blockades are not long tolerated.
This brings us to the Taggers. All sentient races appear to harbor a small minority who feel compelled to go through life marking their way. Cavemen draw ochre depictions of their prey. Roman gladiators scratch graffiti describing their victories on the walls of the Collusion. Young Victorian lovers carve declarations of eternal devotion on stately oak trees. Grimy gangbangers spray weirdly evocative emblems on freeway overpasses. Zyan refugees refueled on a Martian moon and even these non-humans carved "The game is OFF" on a crater's regolith with their mining lasers.
But not in the Strange Worlds' tunnels. The Sweepers are firm but unyielding, and the animosity between Sweepers and Taggers is legendary.
In Nexus city, a memorial plinth commemorates the Sweepers who perished in this eternal controversy. On a nearby walkway, in red epoxy spray, are the words, "They made us do it."