The Boulby Mine, on the North Yorkshire coast, is home to a deep underground laboratory hosting experiments studying research topics including dark matter, climate science, carbon-capture, and the search for alien life. In 2012, I was invited to visit by the Director Sean Paling to investigate its potential for planned experiments…
The bottom of a potash mine is one place where I never expected to end up doing science. When I began my career as a particle physicist, I had a vision that I would be working at large accelerator laboratories in cosmopolitan cities like Geneva or Hanburg. However I am now getting up at 6am, preparing to go to the Boulby Mine in North Yorkshire to join the morning shift riding down in the cage to a depth over a kilometre beneath the surface.
Dr Sean Paling picks me up in Whitby, and we drive out of the town and up the coast, with a fine view of the North Sea and the cliffs where it meets the land. We stop off at a small supermarket to get some sandwiches and a newspaper. This early in the morning, most of the other customers are miners, some already dressed ready to go underground. The mine is one of the few remaining large industries in Yorkshire, employing several hundred people extracting the potash which is turned into fertilizer. It is also the home of the UK’s deepest underground laboratory, the site of many world-leading experiments.
Driving onto the industrial site, we pass between the tall buildings which process the potash and a line of railway trucks ready to transport it to the docks at Teesside. I am issued with fluorescent orange clothes, hard hat, head lamp, safety boots, ear protectors, and self-rescuer—an emergency oxygen source, which everyone venturing down the mine must keep strapped to their waist at all times. We are now running late. Sean rushes me over to the medical room to get an OK from the site medic, repeatedly checking his watch as we approach the departure time. Verified healthy enough to go, we join a group of miners at the top of the main shaft. Pulling our ear protectors into place, we all squeeze into the cage. There is a brief pause before it begins the plunge into the Earth, down the deep shaft into the darkness.
We are going deep underground to get away from radioactivity. The surface of the Earth is continually bombarded with high energy cosmic radiation from outer space. The highest energy particles—cosmic muons—can punch their way deep into the ground, but very few of them make it as far as the Boulby mine. It is dark and cramped in the cage. This is not a job for anyone suffering from claustrophobia. It is uncomfortable, but it is also kind of thrilling. To the miners, it is just another working day, but I am pumped up with excitement at the prospect of another underground science adventure.
It only takes a few minutes to descend into one of the deepest mines in Europe. We emerge and begin the trek along the dark tunnels, kicking the dust with our boots. At the end of our trek, we pass through a small door in a dark niche, and into the antechamber to the laboratory, where we must change outfits again. We swap our dusty boots and hard hats for clean ones, and pull pristine white full-body suits over our orange outfits. The laboratory must be kept clean. Any dirt we bring in will include radioactive contaminants.
Finally we enter the laboratory itself. It is a long room with an orange floor lit by fluorescent strips. An overhead crane runs on rails down the length of the room to lift heavy equipment into place. The room is crowded with racks of electronics, cryogenic tanks, pressurised gas cylinders and all the other equipment required to support the various different types of underground science.
The laboratory was originally founded as a home for dark matter search experiments, run by the UK Dark Matter Consortium—a network of researchers from across Britain. Dark matter was—and still is—one of the biggest unsolved mysteries in modern science, a fundamental research question which bridges astronomy and particle physics. Its importance is hard to understate and this is what has led so many scientists deep underground. Particle physics provides a possible answer to this mystery—that dark matter is made up of a new particle, which interacts only very weakly with ordinary atoms. Searching for these rare interactions requires sensitive detectors, and a deep underground site.
The British-led team pioneered a range of dark matter detector technologies in the 1990s, using scintillation detectors, which record the flashes of light created when particles pass through certain materials, such as liquefied xenon. Boulby was the home of a set of xenon dark matter detectors named ZEPLIN-I to ZEPLIN-IV, each bigger and more sensitive than the last. The ZEPLIN detectors showed the power of this technology in the search for dark matter. However their incredible sensitivity reached a point where they needed an even deeper underground site. The team have since joined a collaboration with researchers in the US, the LUX-ZEPLIN or LZ experiment, deploying the technology pioneered in Yorkshire to the Sanford Underground Laboratory in a gold mine in South Dakota.
The big dark matter experiments moved out, but this was not the end for the laboratory. The unique underground environment turned out to be ideal for many other area of research. Boulby found a new role as a host for other underground science projects. These included the SKY-ZERO experiment—a project to understand the role of cosmic radiation in forming clouds. It is thought that when a muon hits an atom high in the sky, it can trigger the water vapour to condense into a droplet. These particle-induced clouds could have an impact of the climate, but to understand this, we need to study artificial clouds deep underground, away from the muons. Another project uses the few cosmic muons which penetrate this deep, to chart the density of the rock through which they have travelled. This may be used in the future to monitor carbon dioxide stored in old gas fields. Finally there is a programme to study the microbial life in the deep rock layers, and see what this could tell us about life on other worlds.
With a bit of childish imagination, while trekking down the mine tunnels, you could pretend to be an astronaut exploring under the surface of an alien planet. The bare tunnels could easily be taken as part of an underground base on the moon or Mars. It seems like a barren place, devoid of any life except for human explorers. But appearances can be deceptive. It turns out there is a considerable diversity of single-celled life existing in the rocks this deep underground, which date back millions of years. By studying these lifeforms, we can understand the sort of life which might exist on Mars or another planet.
This is the fourth deep underground laboratory that I have worked at. Each one is different. The Gran Sasso and Modane Laboratories are built in motorway tunnels. The Low Noise Laboratory at Rustrel, in the South of France, is in a former Cold War command centre. While every laboratory is different, there are many common aspects. They were all set up to host particle physics experiments, yet they have come to provide a home for a wide range of other projects studying low noise signals in a unique environment. Another thing they have in common is that after setting up experiments, there is a lot of waiting around. At Gran Sasso we had to wait for the bus service which ran from the underground lab out of the motorway tunnel. At Boulby we must wait for the next cage ride up to the surface. This wait does, however, provide an opportunity to talk with the other researchers and learn about the other fascinating experiments going on in these tunnels.
Thanks for Sean Paling and everyone at Boulby for showing me around, and to Babak Abi who accompanied me on my second visit.