This thing might be eyeing me up, but I can’t be sure—it’s got eight of them, and they all point in different directions. It seems impassive in the face of what must be a blinding glare from my head torch—perhaps the first light it has ever seen. Spelungula cavernicola is the biggest spider in the land, but most of it is legs—150mm from claw to lethal-looking claw. The species’ beauty seems such a waste in the dark: this one is ringed with amber bands; somehow limpid, so that my torch seems to shine right through it. I can make out the black bristles that sparsely line the spider’s many femurs, but I have to wait for Neil Silverwood’s gorgeous close-up photos before I can see that they give way on each tibia to shocks of shorter hairs, almost downy, which coat the remainder of its spindly shanks. Its dull yellow abdomen, too, is covered in something like day-old stubble.

I stare into its poker face, trying to imagine how it penetrates this dark; how it pays the preternatural attention it surely must to survive in here. It does have eight eyes, but so do most spiders, and many of them still have rubbish eyesight. The topmost pair are oriented slightly upwards. The pair below peer straight at me. It’s thought these four eyes can render shape and colour: the rest, gazing sideways, are there because spiders don’t have a neck. They can’t just turn their head as we do, so these are attuned to movement, looking for peripheral threats.

I wonder why it has eyes at all; they cost a lot of energy to make, and here in this cavern beneath Honeycomb Hill in the Ōpārara Basin north of Karamea, energy comes in patchy pittances from a threadbare food web. But spelungula—from the Latin “spelunca”, for cave—may not be the committed troglobite its moniker suggests. Arachnologists point out that physiologically, there’s nothing stopping it from scuttling out of this cavern into te ao mārama—the world of light—and doing just fine. Maybe it even comes and goes between the realms, exercising a kind of dual citizenship—it’s just one more thing we don’t know about spelungula.

The sole member of its quirky genus, it sits with four feet in each of the two fundamental taxonomic spider groups. It was first collected in the late 1950s, but wasn’t formally described until 1987. Yet it is ancient. Its ancestors almost certainly stalked the dim hollows of Gondwana. “These creatures were around at the beginning of true spider evolution,” says entomologist Ian Millar. Spelungula is protected under the Wildlife Act, and besides, its bite is reportedly painful. So don’t turn one upside down—just take the taxonomists’ word that its belly sports two pairs of “book lungs” behind the rearmost legs. “That’s an old breathing system,” says Millar. “In the more modern spiders, you never find more than one pair of book lungs, and many species have lost them altogether in favour of more advanced systems.”

That’s because, being open to the elements, book lungs can be a bit leaky: “It’s more difficult to maintain moisture levels with them.” Millar has spent more time in spelungula’s company than most, and he thinks those lungs could be one reason it prefers this muggy gloom, where humidity sits steady at 100 per cent. Wisps of my own breath condense in the torchlight, bearing him out. Honeycomb puts little pressure on evolution—there are no seasons here; it’s almost always 10ºC. Neither is there weather, save for the zephyrs that waft from the system’s 70-odd entrances. In places, these breezes have been sufficient to coax stalagmites to set at a jaunty angle, blowing each calcite-laden drip a degree or two off vertical.

This is about as unchanging as I can imagine any world to be. In a pothole nearby, the bones of a moa lie where it fell maybe 15,000 years ago. The place is a crypt. Palaeontologists have found the remains of 50 species in here. Many were last seen alive centuries ago: nine different moa, and their antagonist, pouākai, the giant eagle; the huge flightless goose; Finsch’s duck; a flightless rail; the flightless coot; the strange owlet-nightjar and the more recently annihilated laughing owl; primeval frogs that no human ever saw. Department of Conservation  (DOC) ranger Graham Quinn takes me to one side to point out the remains of a kākāpō.

There are things entombed here that are older still. In the cave wall, right before my eyes, are dimples of marine molluscs that lived 20–30 million years ago. Back then, the Ōpārara Basin, like most of proto-Aotearoa, lay beneath the sea. These creatures are cemented into the calcified corpses of trillions of their smaller contemporaries—bryozoans, echinoderms, foraminiferans—that lived here in such abundance that their shells covered the seabed in a charnel kilometres deep. They were turned to stone by chemistry and their own colossal weight, to become the monolith geologists call the Nile Formation limestones. Much of it has already eroded away, but these caves lie in a narrow north-south remnant beside the Ōpārara River.

Limestone surrenders to water without a fight, melting in the mild carbonic acid concocted wherever carbon dioxide (abundant in limestone) and water meet. At Ōpārara, rainwater percolates through rampant carpets of moss that lend it still more acid—the river runs auburn with tannins. The basin gets between two and three metres of rain a year; multiply that by a million or so, and you’re left with a hallucinogenic netherworld of caverns, tunnels, tomos, arches, underground springs and streams. Honeycomb Hill is perfect for troglophiles such as spelungula, but so are many other limestone cave systems in the land—Waitomo, Clifden, in Southland, or Punakaiki and Paparoa, just two hours’ drive from here. Yet you’ll find spelungula in exactly none of them, which is a riddle. As far as anyone knows, it lives only here at Ōpārara, a little further north in the Heaphy Valley, and in a few select caves in Golden Bay.

This land had already taken a beating from glaciers when the Pacific Plate crashed into the Australian Plate and sent the whole lot skywards, starting about 15 million years ago. Limestone quickly disintegrates in the elements, so at this point many of spelungula’s former refuges were literally washed away. The few that survive are mostly nestled in the sheltering lee of tectonic thrusts. Even there, the ice ages occasionally choked them with glacial grit, to be later flushed clear again by torrents of meltwater.

The best guess, then, is that spelungula has simply persisted where geology has allowed it to. Perhaps it was never widespread, muses Ian Millar: “This thing may always have been confined to northwest Nelson.” As far as he can tell, the most compelling commonality among spelungula’s disjunct haunts is that they’re all home to cave wētā, its preferred diet.

Few have seen spelungula hunt—Millar suspects “it only needs to get a feed every few weeks or so, unless it’s a female with a load of eggs on board”—but observations of captive spiders show how those front claws are put to lethal use. “First, they use them to grasp and inflict damage on the wētā,” he says. The spider will try to whip the wētā off the substrate quickly, so that it can’t spring away on those muscular back legs (this has been embellished into a backcountry myth that spelungula drops upon its victim Tarzan-style from a height, trailing a silk line it uses to hoist the prey from its stance). “Those claws then mesh with an opposite set of spines on the lower leg,” says Millar. The wētā is now skewered between sharp pincers. The coup de grâce is delivered by a venomous bite, “but by that time, the prey’s probably pretty knackered anyway”.

This one, not fully grown, isn’t far from the cave entrance. I turn off my torch and let my eyes readjust. There’s a little light to see by here, but there are spiders deeper in the cave that presumably hunt in total darkness. How—what—do they perceive here in te pōtangotango, the ineffably black night? Spiders don’t have ears; their world is soundless—sort of. For years, biologists assumed they relied on vibrations transmitted through surfaces to home in on their quarry. But we now know they can also feel soundwaves in the air, picked up by all those hairs—setae—on their legs. In experiments, blindfolded spiders have still been able to capture even flying prey, and adopted different postures, from defensive flinching to predatory aggression, depending on the frequency of soundwaves played to them.

But spiders get more wondrous still: this one is detecting my slightest movements—even the waft of my breath—through trichobothria, sensory cells that pepper its exoskeleton, all linked by a filigree of neurons. There are way more neurons in its periphery even than in its central nervous system. This spider is keeping tabs on me with its skin; attuned, as one study put it, to “absolute sensitivities at the limit of the physically possible”. Spelungula is supreme in its own shadowy, sensory dimension.

If a food chain with only two links can have an apex, then apex predator it is.

It needs no trail of breadcrumbs: no silk dragline, no chemical nor gravitational cues, to find its way. In experiments, other species have retraced blindfolded the exact steps of forays longer than 70 centimetres. The creature follows waypoints stored in its own neural network: it remembers every placement of every leg—each movement recalled in perfect reverse sequence, like a rock climber retreating from an impossible problem. It’s called idiothetic orientation; intel provided by thickets of tiny hairs, microtriches, arrayed in their thousands along the shafts of the bigger setae, like the quills of a feather. Those in turn are clustered about the leg joint, so that when a bunch of microtriches brush against adjoining ones, they send the spider a message, letting it know precisely how far a leg is extended, where each foot falls.

For millennia, these perceptive superpowers have kept spelungula safe from predators, flooding seas, and crushing glaciers, but they’re powerless against regional tourism strategies. The 35-million-year-old limestone of the Ōpārara Basin is riddled with caves. The Ministry of Business, Innovation and Employment (MBIE) sees them, along with the magnificent Ōpārara Arch, as crutches for a hobbling West Coast economy.

In 2016, MBIE floated a tourism plan that touted Ōpārara as an “icon attraction”. Cue expressions of interest (“Delivering a World-Class Visitor Experience whilst preserving the environment for future generations”), feasibility studies, six-figure consultancy fees, development proposals—and derision from the outdoors community at a mooted “moa town” beneath the arch, complete with walkways, light shows, life-size moa sculptures and its own phone app.

In 2018, a $5.7m grant let tourism trusts press on with a more reasoned schedule: this version mostly improved road access, security, toilets and tracks. Mirror Tarn got a viewing platform, and proper steps were set into the approach of a smaller arch, Moria Gate.

Not so long ago, around 11,000 people visited the Ōpārara Basin each year. Many of them also walked through a nearby cave, Crazy Paving. They were unguided, save for two string lines to keep them from trampling its eponymous fractured mud patterns. I try to imagine the soundwaves that must have crashed over spelungula as those people moved through; the pounding of feet, the shrieks of excited children. How did it cope with a daily supernova of torchlight it couldn’t so much as blink against? How did it hope to hunt in that sensory blizzard, with its sole prey scattered by humans?

We can’t know for sure, but in 1998, caver Barry Chalmers wrote in the New Zealand Speleological Society Bulletin that Crazy Paving and nearby Box Canyon Cave had some of the highest numbers of spelungula “found anywhere. On an average day you can see up to 25 spiders doing their thing, within your torch beam only a few feet away.”

DOC’s Graham Quinn has been counting spiders here and in the permit-only Honeycomb Cave since 2008. His numbers, he says, “have bounced around a bit”, but the downwards trend is incontrovertible: by January 2020, he could find only two spelungula. Two months later, the spiders caught a break from an unlikely source: Aotearoa went into nationwide lockdown in a bid to stave off the COVID-19 pandemic. Silence returned to Crazy Paving, for the most part—Quinn kept up his monthly surveys.

Visual counts can be woolly, but after lockdown, that downward trend flattened, then twitched. “I’m not saying that the spider numbers are increasing, but the counts are certainly increasing.” Quinn says that could be down to reclusive individuals returning to more conspicuous sites once the disturbance stopped. But a steady rise in sightings hints strongly that for spelungula, privacy brings prosperity.

In May this year, as part of the Ōpārara upgrade, the access road into the basin was closed for improvements. DOC took the opportunity to simultaneously close Crazy Paving for spelungula’s sake, installing a stout steel grate across the entrance. There’s an access hatch in the middle of it that takes a bit of bastardised yoga to squirm through. Before I’ve even completed the move, ranger Scott Freeman gives a gleeful chuckle: barely a metre inside, there’s a spelungula—not fully grown, but getting there—perched on the wall. As I straighten up, my head torch finds another. Three weeks after the cave closure, Quinn came here and found 13 spiders—his best tally in two years. Today, we’ll see nine.

The rangers are looking, too, for another bellwether: females place their eggs in a woven orb, maybe 20 millimetres across. They make no effort to hide it—it dangles in plain view from a cave roof or shelf on a silken thread. Inside are around 40 eggs. After six months or so, they’ll hatch, and the spiderlings will chew their way out of the orb, clamber up the thread and, with luck, prosper for three years and possibly longer (most other spiders get only 12 months on this earth).

Egg sac counts are a pretty good indication of where the population is headed, and Quinn hasn’t seen one since 2018, when he found just one. Figure in that three-year lifespan and it seems clear that spelungula were going to the cave wall. A few more metres inside Crazy Paving, Freeman spots what the rangers have been hoping for: a ripe egg sac hangs in the beam of his torch. Later, he’ll find another. It took a couple of years of peace and quiet, but the spiders are making a comeback.

Box Canyon cave, next door, will remain open, says Freeman, “so people can still experience a cave environment, and potentially see spiders”. Crazy Paving will be off-limits until June 2023, and Quinn will keep up his monthly counts. Whether the grate stays locked after that will depend on what he finds.

“Closing public conservation land is not something you take lightly,” he says, “but our priority is to protect the spiders.”

Ōpārara might be the seat of the realm of darkness. The fact that some people felt what it needed was a light show speaks to its greatest vulnerability. Humans don’t get the point of these places, we are disinclined to let the dark places of the world be, to leave them as sanctuaries of lightless silence, so that the merest scratch of a wētā’s foot will waken the senses of an ancient and otherworldly predator.