It's another installment of Entropist, a scifi culture column by futurist design maven Geoff Manaugh, author of BLDG BLOG. You stumble on a cave in the mountains of Slovenia. Rumor has it this place inspired Dante's descriptions of Hell in his Divine Comedy. Called the Postojna Jama, it's a real cave. Let's say, then, that you join a group of people milling about at the cave's entrance before you all descend into the deep. At a point that clearly isn't the bottom, you're told to turn around. But why stop? you think, looking ahead into the darkness. Is there something down here we shouldn't see? In an utterly cheesy, but nonetheless enjoyable - even impossible to stop reading - novel called The Descent, author Jeff Long presents us with a very similar premise. It involves nuns and the U.S. military and Himalayan mountaineers and a weird parallel branch of the human species, some rogue sub-race that went literally underground so many tens of thousands of years ago - and is only now coming back into the light.

They're called Homo hadalis. Get it? They're from Hades, "the planet within their planet," as Long calls it - where their refers to the military men who now find themselves confused by this brand new enemy that confronts them from below.

Soon enough, finding more and more of these literally hellish non-humans pouring up from the bowels of the Earth, killing thousands before disappearing again into unlit caverns, the militaries of every nation in the world plan a subterranean invasion. Armed with machine guns, hydroponic agriculture, UV lights, and lots of instant concrete, they head downward. They begin the descent.

Indeed, organized and state-funded, the militaries "approached the subplanet the way America approached manned landings on the moon forty years ago, as a mission requiring life support systems, modes of transportation and access, and logistics."

Vast caverns are mapped. Tunnels stretching clear across the Pacific seafloor are discovered - and, from there, cobwebs of subsidiary tunnels, weaving off into an abyss:

The abyss beneath the Pacific is basalt, which gets attacked every few hundred years by huge plumes of hydrogen-sulfide brine, or sulfuric acid, which snakes up from deeper layers. This acid brine eats through the basalt like worms through an apple. We now believe there may be as many as six million miles of naturally occurring cavities in the rock beneath the Pacific, at an average depth of 6,100 fathoms.

The earth, in other words, is hollow. There are thousands of tiny tunnels, like capillaries, but big enough to walk through - and there is one massive one, a geological superhighway spiking east from the Mariana Trench. It angles toward a nest of smaller caves on the surface as far away as Peru.

As one of Long's characters says:

"Where it goes, we're not quite sure... A profusion of tunnels shoots throughout the Asian plate systems, giving access to the basements of Australia, the Indonesian archipelago, China, and so on. You name it, there are doorways to the surface everywhere."

There are doorways to the surface everywhere - but the traffic moves both ways. Things come up; things go down. One of those doorways is the Postojna Jana, mentioned above, 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; people move between underground base camps and vast instant cities further on, full of klieg lights, ringed with landmines, thriving behind walls of sandbags 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 soldiers 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."

Suddenly, man no longer looked out to the stars. Astronomers fell from grace. It became a time to look inward.

To look into the Earth...

There is a rich vein of subterranean adventure in science fiction, from Jules Verne, of course, to Neil Marshall's recent horror film The Descent and the unwatchably bad The Core - or even the Bible, where we read about the harrowing of Hell, which the Catholic Encyclopedia describes as a "triumphant descent" into the planetary abyss.

I'm tempted to quote Nietzsche. After all, with all this talk of entering into unexplored realms of pressure and darkness, looking into a void that perhaps looks into us in turn, the obvious final question is: Are we prepared for what we'll find?

As Jules Verne himself wrote: "Look down well! You must take a lesson in abysses."