Underland

by Robert Macfarlane

  Sean and I stand up in the warm wind and we follow the barrows, walking past each mound in turn until we have reached the end of the line, the last of the nine. From there we return to the first of the barrows, and we lie again on its slope, talking and not-talking. Beneath us is the earth and the kists it holds, and below that is the limestone and the rifts it holds.

  We lie on the barrow’s turf for so long that, when we leave, I look back and see that we have pressed imprints of our own bodies into the grass of that burial site, leaving outlines of what is to come.

  3

  Dark Matter

  (Boulby, Yorkshire)

  More than half a mile under the earth, in a laboratory set into a band of translucent silver rock salt left behind by the evaporation of an epicontinental northern sea some 250 million years earlier, a young physicist is trying to look into a void.

  He sits watching a computer screen, close to a large silver cube. The cube’s name is DRIFT and it is a breath-catcher. The young physicist is trying to catch the faint breath of a particle wind sent blowing across space from a constellation called Cygnus, the Swan, many light years distant from Earth.

  The young physicist is searching for evidence of the shadowy presence at the heart of the universe: a presence so mysterious that it has thus far engulfed almost all of our attempts either to investigate or to represent it. The name we have given to this presence – which refuses to interact with light, which may not even exist – is ‘dark matter’. And the only place the young physicist can conduct his enquiry is down here in the underland, shielded from the surface by 3,000 feet of halite, gypsum, dolomite, mudstone, siltstone, sandstone, clay and topsoil.

  It is a paradox of his work that in order to watch the stars he must descend far from the sun. Sometimes in the darkness you can see more clearly.

  ~

  In the early 1930s a Swiss astronomer called Fritz Zwicky was studying galaxy clusters through the telescopes of the California Institute of Technology when he noticed an anomaly of extraordinary implications. Clusters are groups of gravitationally bound galaxies, and Zwicky’s work involved measuring the speeds of revolution of individual galaxies in their orbits around the core of the cluster, in order to weigh the cluster as a whole. What Zwicky observed was that the galaxies were revolving much faster than expected, especially towards the outer reaches of the cluster. At such speeds, individual galaxies should have broken their gravitational hold on one another, dispersing the cluster.

  There was, Zwicky determined, only one possible explanation. There had to be another source of gravity, powerful enough to hold the cluster together given the speeds of revolution of the observable bodies. But what could supply such huge gravitational field strength, sufficient to tether whole galaxies – and why could he not see this ‘missing mass’? Zwicky found no answers to his questions, but in asking them he began a hunt that continues today. His ‘missing mass’ is now known as ‘dark matter’ – and proving its existence and determining its qualities is one of the grail-quests of modern physics.

  How, though, to hunt for darkness in darkness? How to seek a substance that has mass and therefore exerts gravity, but that does not emit light, reflect it or block it? Since Zwicky, the evidence for the existence of dark matter has been gathered largely by inference: the detection not of the matter itself, but of its presumed influence on luminous entities, observable objects. To perceive matter that casts no shadow, you must search not for its presence but for its consequence.

  It is now known, for instance, that dark matter affects the rotation curves of spiral galaxies, causing all bodies within such a galaxy to revolve at comparable rates, regardless of their distance from the galaxy’s gravitational centre. It is also known that dark matter bends light as it passes around a galaxy, causing what is referred to as ‘gravitational lensing’. Mass curves space, Einstein showed in his General Theory of Relativity, and light follows those curves of space – as when it passes around a massive entity such as a galaxy. But just as Zwicky’s galaxies rotated too fast, so light also bends too greatly to be due only to the visible components of a given galaxy. There must again, therefore, be more mass than that which can be seen. This imperceptible, space-curving, light-lensing massive presence that surrounds a visible galaxy is known to astrophysicists as a ‘dark-matter halo’.

  What these observations and others like them suggest is that only around 5 per cent of the universe’s mass is made of the matter we can touch with our hands and witness with our eyes and instruments. This is the matter of stone, water, bone, metal and brain, the matter of which the ammoniacal storms of Jupiter and the rubble rings of Saturn are made. Astronomers call this ‘baryonic matter’, because the overwhelming share of its mass is due to protons and neutrons, known to physicists as ‘baryons’. A little over 68 per cent of the universe’s mass is presumed to be made of ‘dark energy’, an enigmatic force that seems to be accelerating the ongoing expansion of the cosmos. And the remaining missing 27 per cent of the universe’s mass is thought to be made up of dark matter – the particles of which almost wholly refuse to interact with baryonic matter.

  Dark matter is fundamental to everything in the universe; it anchors all structures together. Without dark matter, super-clusters, galaxies, planets, humans, fleas and bacilli would not exist. To prove and decipher the existence of dark matter, writes Kent Meyers, would be to approach ‘the revelation of a new order, a new universe, in which even light will be known differently, and darkness as well’.

  Dark-matter physicists work at the boundary of the measurable and the imaginable. They seek the traces that dark matter leaves in the perceptible world. Theirs is hard, philosophical work, requiring patience and something like faith: ‘As if’ – in the analogy of the poet and dark-matter physicist Rebecca Elson – ‘all there were, were fireflies / And from them you could infer the meadow’.

  Presently, the particle thought most likely to be the constituent of dark matter is known wryly as a WIMP – a weakly interacting massive particle. What we know of WIMPs suggests that they are heavy (up to more than a thousand times the weight of a proton), and that they were created in sufficiently vast quantities in the seconds after the birth of the universe to account for the missing mass.

  WIMPs – like neutrinos, nicknamed ‘ghost particles’ – have scant regard for the world of baryonic matter. WIMPs traverse our livers, skulls and guts in their trillions each second. Neutrinos fly through the Earth’s crust, mantle and solid iron-nickel core without touching a single atom as they go. To these subatomic particles, we are the ghosts and ours the shadow-world, made at most of a diaphanous webwork. The great challenge faced by physicists has been how to compel such elusive particles to interact with experiments; how to weave a net that might catch these quick fish. One of the solutions has been to go underground. Subterranean laboratories have been established around the world, dedicated to the detection of evidence that a WIMP or a neutrino has briefly interacted with baryonic matter. The experiments under way in these deep-sunk laboratories are all forms of ghost hunting, and they are located far underground because the surrounding rock shields the experiments from what physicists call ‘noise’.

  Noise is the trundle of everyday particles through the air, the din of the ordinary atomic world going about its business. Radioactivity is deafening noise. Cosmic-ray muons are noise. If you wish to listen for sounds so faint they may not exist at all, you can’t have someone playing the drums in your ear. To hear the breath of the birth of the universe, you must come below ground to what are, experimentally speaking, among the quietest places in the universe.

  Half a mile underground in an abandoned mine in Japan, set in a chamber of 250-million-year-old gneiss, a stainless-steel tank holds 50,000 tons of ultra-pure water. Watching the water are 13,000 photomultiplier tubes, forming a compound eye. The eye is looking for tiny flashes of blue light. These flashes are Cherenkov radiation, produced when an electron moves faster than the speed of light in water
. Electrons reach such speeds when an atom is – occasionally – struck by a neutrino, the impact scattering the atom’s electrons at velocities in excess of the speed of light. These scattered electrons are called ‘annihilation products’, and if those electrons are scattered in water then they briefly create a luminous blue cone around them as they move. The compound eye of the photomultiplier tubes therefore watches for trebly displaced evidence of the ‘ghost particles’: not the neutrino itself, or the atom it has struck, or the electrons it has dispersed, but the blue aura left by that ghost-struck atom – annihilation’s afterglow. This buried chamber of gneiss is called an ‘observatory’, for although it is deep underground it is in fact scrying the stars: among its many other tasks is keeping watch for supernovae in the Milky Way.

  Deep in a worked-out open-cut gold mine in South Dakota, super-cooled xenon is held in a six-foot-tall vacuum vessel, surrounded by 71,600 gallons of deionized water contained in a welded steel tank and watched by photomultiplier tubes for the displacement of a single photon and a single electron brought about by the strike of a WIMP. Xenon, a noble gas, has large atoms. When xenon is very cold it is very dense; those large atoms huddle together, thus presenting a greater cross-section to incoming particles and optimizing the chances of WIMP strikes. In a landscape where the earth was once raked and gouged in search of a highly valued rare metal, the search is now ongoing for a substance that is plentiful beyond imagination and of no worth at all.

  And near the little village of Boulby on the Yorkshire coast, in a salt cavern far below the headworks of a potash and rock-salt mine that commenced operation in 1973, a dark-matter detection experiment is presently under way that is known by the acronym DRIFT: directional recoil identification from tracks.

  ~

  Neil Rowley unrolls his map of the underland on his desk, and places four chunks of rock on its corners to keep it flat, naming each as he puts it down: sylvite, halite, polyhalite, boracite. He smooths the map out with his hands, working from the centre to the edges. Neil is a mine-safety specialist. He has worked in coal; now he works in potash. He likes W. H. Auden, he likes maps and he loves mining.

  Neil’s map records the roadways and refuge chambers of the Boulby mine. At first glance, it looks to me like the wings of a dragonfly, intricately veined and structured. Slowly my eyes key into its codes.

  The north-east coastline of England is present as a faint grey line running across the map from north-west to south-east: a surface irrelevance, shown chiefly for purposes of orientation. At Boulby itself, two circles signify the two shafts that plunge into the bedrock, giving access to the tunnel network. From that centre point the tunnels fan out to the north-east and the south-west, forming the wings of the dragonfly. To the south-west, they spread under moor and dale, deep into North Yorkshire. To the north-east they spread under the North Sea, running out beyond the shipping lane and into open ocean.

  This network of tunnels and roadways is collectively known as ‘drift’. There are more than 600 miles of existing drift burrowed into the soft bands of halite (salt) and sylvite (potash) that stretch below sea and land, out to the mining faces where – every hour of every year – men and machines claw tons of potash from the seams, duct the potash onto hoppers and start the journey of this buried residue of a Permian sea up to the world’s crop fields, where it will be spread as a fertilizer in both of the Earth’s annual two springs, returning vital potassium to the growing cycle.

  As the land below the Mendips holds a water-made labyrinth, so the land below Boulby holds a human-made maze. I have come from rift to drift.

  On Neil’s map, red lines signify drift cut through salt, black lines signify drift through potash. Yellow squares mark refuge chambers, dug into the side walls of the tunnels and armoured against heat by polyfoam outer walls. In the case of collapse or fire at depth, these are the fall-back sites.

  At the tips of the wings – far under the sea and far under the moors respectively – thin green threads lick out. These are the lateral boreholes being drilled by mine geologists to test the lie and integrity of the deposits ahead of the workface. The information they return will determine the future directions of the mining, the future spread of the wings.

  ‘You need to understand that the tunnel network is on a tilt,’ says Neil, drawing his finger across the map, from one end of the dragonfly’s wing to the other. ‘The drift tilts because the deposit tilts. The tunnels follow the potash, and the potash strata are inclined.’

  Inland the potash deposit runs deeper, reaching a maximum depth of around 4,500 feet at the outermost limit under the moors. Seawards, it rises to a minimum depth of around 2,600 feet at the outermost point beyond the shipping channel. A temperature gradient follows the depth gradient. At 2,600 feet the air temperature is 35°C. At 4,500 feet it’s 45°C. In both places the geothermal heat is so intense, and the moisture content of the air so low, that sweat evaporates before it can even be seen. Dehydration is rapid. For the miners it is like labouring in the Sahara at noon, in darkness.

  ‘The men all carry cool-boxes with four litres of chilled water per shift,’ Neil says. ‘They have rehydration timetables throughout their shifts. Got to keep drinking. Much safer.

  ‘Come on – let’s see if we can catch a lift down there, find some dark matter, then we’ll make the long drive out to the mining face under the sea.’

  Ear defenders on. Respirator hooked at the belt. Numbered bronze triangle in pocket as proof of entrance: Don’t lose it now, you won’t be allowed out . . . Yellow cage door clangs shut, cage starts its drop, steady but still stranding the stomach. Roar of the fan-house fading away, cage speeding up. Halfway down a shudder and blast as the other cage crosses on its way up, squeezing air between cages with a crash-whoosh like two trains passing in opposite directions. Slow, slow, slow, bump, stop, cage door clangs open – and voices are yelling, ‘Ears off, lights on! Ears off, lights on!’

  Rock dust swirls in the air, thick enough to taste, salty on the tongue.

  Black mouths of drift lead away under the ocean, into the Permian.

  An airlock in a wall opens into a laboratory.

  ~

  The young physicist sits at his computer, watching for signals from Cygnus. His name is Christopher Toth, and his white lab coat is too big for him. Christopher speaks with calm clarity. His manner is modest, gracefully gentle, and I wonder if this comes in some way from spending your days thinking through time so deep it stretches to the birth of the universe.

  Along the walls of the laboratory, at intervals of every fifteen feet or so, black-and-yellow warning tape marks the outlines of what look like potential doorways, rising only to thigh level. Above each taped outline, a long-handled axe with a splitter blade is held in two hooks.

  Salt has very low gamma radiation. Salt is a good insulator. Salt is radio-pure. Salt is an excellent substance in which to encase yourself if you want to study weakly interacting massive particles. But salt is also highly plastic. Salt flows over time. It creeps around. It sags. If you cut a chamber out of a seam of halite with 3,000 feet of bedrock above it, that chamber will slowly distort. The ceiling will dip, the sides will bulge. Gravity wants that space back. So the scientists working in the Boulby laboratory know they are operating in a temporary zone, with limited years of safe life. Deep time must be studied fast.

  ‘Those are your emergency exits in case of a sudden slump in the halite,’ says Christopher, mimicking the hand gestures of a flight attendant explaining the safety protocols, and pointing to the doorways marked with warning tape, ‘here, here . . . and here. If the lab begins to collapse, you grab an axe, hack your way through the lab wall, then hack your way out through the salt to safety.’

  He pauses, smiles. ‘Well, that’s the theory, at least.’

  Several different kinds of underground experiment are presently taking place in the laboratory. One assays rock samples in order to research techniques for the long-term burial of radioactive waste. Anothe
r investigates a technology known as ‘muon tomography’, which makes use of highly penetrating charged particles (muons) produced by cosmic rays from space. Because of their ability to pass deep into rock, muons allow sunken structures such as the interiors of volcanoes and the hollow hearts of pyramids to be perceived. Muons offer a way of seeing through stone. These are all remarkable experiments. But the jewel in the experimental crown of the Boulby laboratory is DRIFT.

  Christopher walks me towards a large object located at one side of the laboratory. ‘This is my underground crystal ball,’ he says – flourishing his hands like a magician revealing a trick – ‘also known as the Time Projection Chamber.’

  From the outside, the magnificently named Time Projection Chamber is disappointing to look at. Black bin liners are taped scruffily around a large metal-clad box.

  ‘I see that bin bags make up the vital outer layer of your crystal ball,’ I say.

  ‘You mock,’ replies Christopher, ‘but duct tape and bin bags have proved crucial to more scientific breakthroughs than you’d imagine.’

  He explains the experiment to me. ‘We know dark matter is massy. Massively massy. So its particles, even though they’re invisible to us, have mass, and if they have mass then they must at least occasionally collide with particles we can see. These collisions send nuclei scattering. Our first goal with DRIFT is to detect these collisions, and follow the nuclei as they scatter.’