Triptych image: A fresco detail of Hell from the medieval church St. Nicolas in Raduil Village, Bulgaria, Anton Lefterov

IN APRIL 2013, Italian archeologists working in southwestern Turkey announced that they had discovered an entrance to Hell. Excavating the ancient Phrygian city of Hierapolis, now known as Pamukkale, they came across the remains of a cave with inscriptions to the god Pluto. This portal, according to one of the archeologists, Francesco D’Andria, was the same one described by Greek geographer Strabo, who, 2,000 years ago, wrote of a space so “full of a vapor so misty and dense that one can scarcely see the ground. Any animal that passes inside meets instant death. I threw in sparrows and they immediately breathed their last and fell.” According to D’Andria, the cave, once excavated, was still every bit as deadly as in Strabo’s time: “We could see the cave's lethal properties during the excavation,” he told Discovery News. “Several birds died as they tried to get close to the warm opening.”

It’s true that, since antiquity, dying birds have been a telltale sign that one is approaching the threshold of the Underworld. In the Aeneid, Virgil similarly describes “the foul-smelling gorge of Avernus” as a place that kills any bird unfortunate enough to fly over:

There was a vast cave deep in the gaping, jagged rock,
shielded well by a dusky lake and shadowed grove.
Over it no bird on earth could make its way unscathed
such poisonous vapors steamed up from its dark throat
to cloud the arching sky.

This avian-unfriendliness is right there in the name: Avernus, from the Greek aornos, “birdless.”

But if birds fall dead from the sky over the “foul-smelling gorge of Avernus,” it’s not, as Virgil and other ancients assumed, because it smells foul; in fact, it’s pockets of odorless carbon dioxide escaping from the earth that kills them. The stink, however unpleasant, is unrelated: it’s sulfur. Carbon dioxide wasn’t identified until the 17th century, so it’s understandable that Virgil wouldn’t have known that the sulfur smell emanating from caves around Avernus wasn’t directly related to the lethal gas that wiped out the local fauna. Nor was he alone in assuming that Hell, wherever it may be, must reek of sulfur — the association is thoroughly ingrained in Christian culture as well, as evidenced by the well-worn cliché “fire and brimstone” (the latter being the nontechnical term for sulfur).

Is this relationship more than pure coincidence? This is one of several questions that Salomon Kroonenberg seeks to answer in his very strange, compelling, and aptly titled Why Hell Stinks of Sulfur: Mythology and Geology of the Underworld. A geologist by training, Kroonenberg takes Homer, Virgil, and Dante as his guides as he attempts to mediate between what these epic poets described of the worlds beneath our feet and what we now know about them. A “look at hell from a geologic perspective,” he calls it: fieldwork from the abyss. Along the way, he offers geologic explanations of the mythology of the Underworld, explaining how real geologic events filtered into our ignorant consciousnesses, and how past cultures have been able to shape natural phenomena they barely understood into rich stories of life and death, Heaven and Hell, above and below.

Like many scientists writing for a popular audience these days, Kroonenberg is an evangelist for his cause. We are accustomed to environmental efforts to save the oceans and the air, and the multitude of animals that swim, fly, or crawl about the earth, but we tend not to treat the actual earth beneath our feet with the same compassion or respect. If anything, it’s where we hope to put all the pollutants we don’t know what else to do with: where we plan on burying our nuclear waste, and where we hope to trap the greenhouse gases we want to get out of our atmosphere. But perhaps, Kroonenberg suggests,

we should no longer see the world beneath our feet as a black box in which to dig tunnels, a supermarket for raw materials, a carpet to sweep our rubbish under, or the final resting place of the dead, but as an irreplaceable archive and a living ecosystem whose riches we have as yet hardly been able to fathom.

But rather than simply scolding us for our ignorance, Kroonenberg takes us on a voyage into the depths. He does this “not because of a macabre sort of ‘oltretomba’ [Italian for ‘Underworld’] tourism, but to show how much fear and imagination have triumphed over science, and how much imagination we need even now to picture what it must look like down there.” What elevates Why Hell Stinks of Sulfur above the wealth of recent environmental cris de coeur is the strange dialogue between science and literature that Kroonenberg develops. Contrary to what one might expect from a geologist tackling classical mythology, he is not out to simply disparage the ancient superstitions and leave it at that. It’s true that his book aims, as he writes early on, “to show how much fear and imagination have triumphed over science,” but he also wants to remind us how little we actually know about the earth’s core, and “how much imagination we need even now to picture what it must look like down there.” Throughout the book, Dante plays Virgil to Kroonenberg’s Dante, as he freely adapts the Italian’s schema of the Inferno with chapters like “Charon’s Ferry,” “Avarice,” “The River of Tar,” “The Lead Cloak,” and so on — each acting as a point of entry for a specific human attitude towards the underworld.

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Admittedly, the start of Why Hell Stinks of Sulfur isn’t promising. Kroonenberg spends the first few chapters taking literally the glimpses of geologic information he finds in biblical and classical texts on the Underworld. He travels to the limestone quarries outside of Jerusalem in a vain and almost farcical attempt to find in them a gate to Hell, as though the Bible could be read as functionally as Lonely Planet. He follows in the footsteps of the 19th century embryologist Karl Ernst von Baer, who, in his spare time, devoted himself to proving that the Odyssey contained literal geographic clues that could be conclusively mapped. The description of the bay of the Laestrygonians, where Odysseus supposedly reached Hades, “so closely resembles that of the bay of the Balaklava” in Crimea, von Baer wrote, “that it cannot be a coincidence.” Kroonenberg, traveling to Crimea himself, of course finds no conclusive evidence of Hell, other than the traces of an infernal oil spill in 2007: 30,000 birds died as a result of the wreck of the Russian oil tanker Volgoneft-139, transforming the Strait of Kerch into a kind of Avernus itself. The same storm that wrecked the oil tanker also sunk three other ships carrying, between them, 7,000 tons of sulfur.

These circumstantial echoes aside, it’s not at all clear in these opening chapters what Kroonenberg’s gambit is, other than to prove, once and for all, that the ancient heroes and poets did not actually descend into Hell. It is not until he makes it to Lake Avernus, which Virgil named as the entrance to Hades in the Aeneid, that his research really begin to pay off. Avernus is a crater lake, formed from an extinct volcano, and though it holds no dangers for birds today, there are nearby sulfur springs that have been renowned for centuries for their abilities to heal everything from scabies to erectile dysfunction. Nearby is the infamous Grotta de Cani, or “Cave of the Dogs” — so named because, while a human standing upright within it can breathe normally, dogs quickly die (as would anyone who stooped to their height). The reason for this strange discrepancy would not be discovered for years: the lower half of the cave consists of pure, heavy carbon dioxide, odorless and fatal, while the upper half is filled with breathable oxygen.

Both gases, as it turns out, are released by subterranean magma hitting limestone, and are common byproducts of volcanic formations such as the caldera that eventually became Lake Avernus. People who lived above large limestone deposits — the people of the Mediterranean included — were thus used to encountering subterranean pits that emitted either deadly carbon dioxide or odiferous sulfur. It is for this reason, Kroonenberg argues, that the stench of sulfur begins to emerge as a key component of the stories such people told about Hell. When water beneath the surface of the earth gradually erodes limestone until it can no longer support its own weight, sinkholes can unexpectedly erupt — yet another frequent “hellish” occurrence for dwellers in the Mediterranean region. To answer the question embedded in Kroonenberg’s title, then: Hell stinks of sulfur because the biblical and classical writers who described it first all lived above limestone.

Which is to say: perhaps W.H. Auden was speaking for all of us when, in his 1948 poem “In Praise of Limestone,” imagining “bodies rising from the dead” and a “life to come,” he wrote: “what I hear is the murmur / Of underground streams, what I see is a limestone landscape.” The curious properties of limestone — it is dense, porous, and chemically reactive — create not only sulfur springs, but many of the other specific geologic features that find their way into Hebrew, Greek, and Roman texts about how to reach the realm of the Dead. “Who knows,” Kroonenberg concludes, “perhaps the Underworld would have looked very different if classical culture had developed around the North Sea.”

Just as these ancient societies spread their ideas of philosophy, mathematics, art, and government throughout the world, so too did they spread their assumptions about geology, even to regions with vastly different soil and rock compositions. As Western culture spread outward from the Mediterranean into northern Europe, Asia, and the Americas, this same conception of Hell traveled with it, even though it belonged specifically to that original geologico-cultural formation. Assessing various early modern depictions of the earth and of the Underworld, Kroonenberg notes that they all share a certain basic geologic assumption: the ground below is made from limestone. These classical depictions of Hell formed the basis not just for later mythologies, but for later geologists as well. Describing the English rector Thomas Robinson’s Anatomy of Earth (1694), Kroonenberg muses:

You might wonder where all these collapsing layers of earth — and all that subterranean water and those gradually widening subterranean courses — come from. They are all typical features of karst landscapes in limestone: the springs, the ponors, the cracks in the rocks made ever wide by dissolution, the dolines and the poljes. It is as though, for 2,000 years, the limestone regions of Greece were used as a model for the origins of the Earth.

Which is not to say that descriptions of Hell haven’t evolved, though: Kroonenberg points out, they have evolved according to the specific geological logic most familiar to those describing it. When, in 1758, the Swedish mystic Emanuel Swedenborg described his vision of “countless hells … all divided and regulated with the utmost exactness and congruity … communicating with one another, some by passages and some by exhalations,” hells “under every mountain, hill, rock, plain and valley,” his descriptions uncannily recall the massive copper mines in the Swedish town of Falun, which he had recently visited.

The argument that emerges from Kroonenberg’s excavations, then, is that our depictions of Hell are the result of a sort of geologic unconsciousness. Hell represents the return of the geological repressed: sulfur, carbon dioxide, and other unwelcome elements of the earth bubble up, become distorted by human misperception, and are transfigured into images of perdition. While we may occasionally notice the way that the animals and plants around us edge into our subconscious and populate our mythologies, we are far less aware of the similar contributions made by rock formations and invisible gases to human cultures and myths. Yet we carry the history of Earth’s geology in our dreams.

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Having established the strong connection between myth and geology, Kroonenberg then proceeds, in the main section of his book, along the path trod by Dante and Virgil. In chapters named after various stations in L’Inferno (“The City of Dis,” for instance, discusses attempts to live and work underground, while “Avarice” focuses on attempts to extract mineral wealth), he draws parallels between how we treat the world beneath us (both past and present), and how poets and writers have transfigured this treatment through allegory and symbol. From this point onward, Why Hell Stinks of Sulfur moves from anecdotes about geologists chewing loam in order to discern its clay content, to descriptions of modern day Infernos like South Africa’s gold mines, which are so deep underground (four kilometers) that workers wear vests filled with ice to withstand the punishing heat. Nor is damnation the only thing he finds underground. Kroonenberg notes that miners’ shrines to Saint Barbara, patron saint of protection against spontaneous explosions, are common. He also visits the bizarre Salt Cathedral of Zipaquirá, Colombia, a church built in an abandoned salt mine 160 meters below the surface of the earth.

Offering a series of fluid translations between these radically different discourses, Why Hell Stinks of Sulfur resembles the art historian Lawrence Weschler’s Everything Rises: A Book of Convergences more than it does a work of popular science. Like Weschler, Kroonenberg has a knack for drawing out startling visual comparisons: a Gustave Doré illustration of Dante’s The City of Dis, for example, bears a striking and unsettling resemblance to a contemporary photo of Chinese mine workers being lowered into a coal shaft in Xinjiang. Drawing not just from the usual suspects like Virgil and Dante but from figures as wide-ranging as Athanasius Kircher and Jules Verne, Kroonenberg’s book represents the best kind of scholarship: Catholic in its breadth and rigorous in its depth.

It remains puzzling, then, that Why Hell Stinks of Sulfur includes no mention of what is perhaps the most fraught contemporary geologic issue: hydraulic fracturing, or “fracking,” in which shale is broken up to release natural gas, a process which has been connected to (among other problems) increased earthquakes in areas not normally associated with faultlines. Because Kroonenberg, a Dutchman, is not from the United States, where this is a front-burner political issue, it may be unfair to fault him for omitting any discussion of the process. But it would have been something to see what his strange and curious intellect might have made of the debate on fracking: Where in Dante’s nine circles would we find the closest parallel? What mythical beast might come closest to representing the hydraulic well?

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As the Voyager spacecraft reaches the outer bounds of our solar system some 11 billion miles away, and telescopes like the Hubble peer ever further into the deepest reaches of space, we remain almost entirely ignorant of the universe beneath our own feet. As Kroonenberg notes, while the earth is 12,000 kilometers in diameter, the closest we’ve been able to get to its core is one thousandth of that distance. The deepest hole ever drilled was in Russia, close to Norway’s border, on the Kola peninsula. The plan was to drill 15 kilometers deep, but temperatures were so hot (80 degrees Celsius higher than originally expected), that the 19-year project reached its maximum depth at 12,262 meters in 1989. At the time, reputable news sources such as the Trinity Broadcasting Network and the Weekly World News reported that the project had been stopped, not due to extreme temperatures, but because the team had inadvertently drilled a hole to Hell — a legend that’s proved so stubbornly persistent that it spawned a Wikipedia page documenting its history.

As Kroonenberg relates, some tried to trace where this groundless story had originated, but such attempts at enforcing the truth were and will always be futile. “[F]or some Christians,” he concludes, “it was proof that hell — and therefore God — existed. They even thought that they could hear a church bell, and created their own clangor to go with it.” The need to read geology as mythology is as old as human culture itself, and it is perhaps integral to who we are — a means of negotiating the divide between our feet and the ever-shifting ground beneath them.

The earth, on the other hand, remains unimpressed by our anxieties and our mythologies, and perhaps even by our destructive incursions into its crust in search of precious ore and metal. It’s not that we’re destroying the planet. It’s that our most egregious acts will leave, at most, mysterious scars that future civilizations — future species — will some day discover and vainly try to decipher. Unable to make any more sense of our actions than we ourselves have, they will perhaps resort to myth, which remains the best bridge between conscious memory and that much older, more alien time of the rocks and magma below.

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Colin Dickey is the author of Cranioklepty: Grave Robbing and the Search for Genius and Afterlives of the Saints: Stories from the Ends of Faith.