In general, nocturnal of subterranean species have less interest in the visual world, and depend on other sensory modalities. Visual capacity is a continuum, with humans falling somewhere in the center.

There are gateways to the surface everywhere.

The traffic moves both ways. Things come up: things go down. One of those gateways is the Holland Tunnel with the implication that Dante had literally been describing Hell, having seen its subsurface chambers.

Soon the Army Corps of Engineers gets involved. "They were tasked to reinforce tunnels, devise new transport systems, drill shafts, build elevators, bore channels, and erect whole camps underground. They even paved parking lots three thousand feet beneath the surface. Roadways were constructed through the mouths of caves.

It takes days at a time to get anywhere; organisms move between underground base camps and vast instant cities further on, full of klieg lights, ringed with landmines, thriving behind walls of sandbags, Jersey and Texas concrete barriers and fortified machine gun nests. There are outbreaks of tropical cave disease and claustrophobia and there is something else down there, THAT ENEMY TWIN of the human race.

Everywhere the descending blind organisms find evidence of primitive occupation at the deeper levels, down amidst overwhelming pressures beneath continents and beneath the sea.

Of course surface dwellers want to explore; they want to see where the tunnels lead, to go out to the edges of the earth by going into the earth. Into Hell?, some characters innocently ask. NO, Not Hell: into an upper lithospheric environment, we read. "An abyssal region riddled with holes".

It's time to look inward!.