Underground laboratories are a place of work familiar to many particle physicists. Experiments like dark matter searches and neutrino detectors must be run deep beneath the surface in order to avoid the cosmic radiation bombarding the Earth. To accommodate such projects, many facilities have been established, such as the Gran Sasso laboratory in a motorway tunnel in central Italy, the SNO facility in an Ontario nickel mine, and the world-leading series of neutrino experiments at Kamioka in Japan. These locations have become familiar names to many of us.
However perhaps my favourite underground laboratory is not quite so well-known. This is LSBB – the Laboratoire Souterrain à Bas Bruit, or Low Noise Underground Laboratory, at Rustrel in the French region of Provence, north of Marseille. The unique environment of this site is not so much its deep location (at just over 500m, it is shallower than most others), but its low noise property. This is a super quiet place, both in the acoustic sense, and in other ways.
Its history goes back to the Cold War. In 1960 the French Republic detonated an atomic bomb in the Algerian Sahara, and became the fourth nuclear power. At this point work was already underway on the establishment of the force de frappe triad of air, land and sea-based systems for nuclear weapons delivery, including 18 sol-sol balistique stratégique missiles sited in silos across the Plateau d’Albion mountains in the region of Vaucluse. The design necessitated an underground command centre, able to withstand a direct nuclear strike, to allow for retaliation and make the system a credible deterrent.
Therefore a horizontal tunnel was dug into the mountain outside the village of Rustrel. At the deepest point, 512m under the summit, a steel capsule was built into a concrete lined cavern. Inside this, a small room was mounted on huge shock absorbers. For over twenty years, there were always two officers here, strapped into chairs at a command panel, such that even in the event of a strike, they would be shielded from the shock wave. This continued until the end of the Soviet Union, when the missiles were scrapped and the silos filled in. However the Capsule at Rustrel would find a new role.
At this time, Professor Georges Waysand was looking for a suitable site for his dark matter detector. Dark matter detectors aim to reveal the interactions of hypothetical particles believed to dominate the mass of the galaxy, but as their interaction rate with ordinary matter is so low, they have never been seen. If the theory is correct billions pass through the Earth every day. To test the hypothesis, you need a super-sensitive detector, and a location away from all other particles, which means going deep underground.
Professor Waysand’s team had another requirement. Their detectors were superheated droplet detectors, which use an array of microphones to listen for the sound of bubbles collapsing when a particle interacts in a specially-prepared gel. This needed somewhere very quiet. When he heard that the Air Force had a suitable site, Waysand arranged to take over the underground complex and LSBB was established.
This turned out to just the beginning of a long, and ongoing scientific adventure. Waysand was ideally qualified to start this project, as over his long career, he had dabbled in many different areas of science, from particle and astrophysics, to superconductivity and earth sciences. He was therefore able to spot the other exciting possibilities of a low noise underground site. He realised that the thick steel walls of the LSBB Capsule would keep out the magnetic noise, but the interesting magnetic signals generated by space weather processes in the ionosphere— the region at the top of the atmosphere, where radiation strips the electrons off atoms forming ions—could be detected with a suitable magnetic sensor system.
The laboratory was also kitted out with a network of seismometers to listen to the sound waves from distant quakes and other effects. The combination of magnetic and seismic sensors allowed some interesting seismomagnetic studies. When the ground shakes after an earthquake, it sends a sound wave up through the atmosphere. On hitting the ionosphere, the vibrating ions produced a magnetic signal.
The artificial tunnels are also a fascinating place for hydrogeology research. Mapping the flow of underground water from the mountain-top, where the rain falls, to springs on the lower slopes is a challenging task, even with the latest technology. The water flows through a chaotic network of karstified limestone, eroded by natural acidity over millions of years. Climate change and a growing demand for water mean it is increasingly important to manage the aquifer in the most efficient way. The Rustrel lab provides a rare opportunity to monitor the groundwater from the inside of the mountain. Wherever water drips into the tunnel, the rate is measured, and samples can be taken to monitor the traces of dissolved ions. Looking at this allows the research team to improve their models of where the water flows, and test new instrumentation to monitor this.
Many underground laboratories originally established for dark matter have since sought to diversify into geophysics. The Rustrel laboratory has embraced inter-disciplinary science more than any other I have visited. Every two years, the laboratory hosts a small meeting on Inter-Disciplinary Underground Science and Technology – iDUST. This is one of my favourite scientific conferences where you can hear talks ranging from the search for dark matter and measurements of underground radioactivity to possible earthquake precursor signals and new methods to search for fossil aquifers. A range of applications which could never have been imagined when the complex was first built.