Characterisation of the Bedretto Underground Site for Fundamental Physics Experiments

The Quietest Place on Earth (and Why We Need It)

Imagine trying to hear a single whisper in the middle of a bustling rock concert. That is essentially what scientists face when they try to study the universe's most elusive secrets. Deep underground, far away from the chaos of the surface, there are experiments looking for things that almost never happen: particles of "dark matter" that might make up most of the universe, or rare nuclear decays that could explain why we exist at all. To catch these ghostly events, scientists need a laboratory that is as quiet as possible.

Why is the surface so noisy? First, the sky is constantly raining down high-energy particles called cosmic rays, which crash into everything and create a background "static" that drowns out delicate signals. Second, the ground itself vibrates with every passing car, train, and ocean wave. Third, the rocks around us naturally glow with a faint, invisible radiation from elements like uranium and radon. To solve this, scientists build deep underground labs. The massive pile of rock above acts like a giant shield, blocking the cosmic rain and dampening the vibrations. The deeper you go, the quieter it gets. But finding the perfect spot—one that is deep enough, quiet enough, and easy to get to—is like finding a needle in a haystack. This is where a new contender enters the game.

The Search for the Ultimate Underground Hideout

A team of researchers has just finished a massive "health check" of a new potential home for science: the Bedretto tunnel in the mountains of Ticino, Switzerland. Think of this tunnel not just as a hole in the ground, but as a potential "quiet room" for the most sensitive instruments ever built. The team wanted to know: Is this place quiet enough to hear the universe whisper?

They set up shop about 3.5 kilometers into the tunnel, a spot called TM3500, which sits under more than 1,400 meters of solid mountain. To test the site, they acted like cosmic detectives, measuring everything from the rain of particles hitting the walls to the hum of the earth itself.

The Cosmic Rain Check
First, they looked at the "cosmic muon rain." Muons are like tiny, high-speed bullets from space that can punch through almost anything. On the surface, they hit us constantly. The team set up a telescope made of plastic panels to count them. The result was spectacular: at the Bedretto site, the muon rain was suppressed by a factor of one million compared to the surface. It's as if they moved from a hurricane to a gentle breeze. This level of quietness puts Bedretto in the same league as the world's best underground labs, making it a prime candidate for experiments that need to be shielded from cosmic noise.

The Rock's Radioactive Glow
Next, they checked the walls themselves. Rocks aren't perfectly silent; they contain natural radioactive elements that emit gamma rays and neutrons, which can look like fake signals to sensitive detectors. The team took samples of the granite from the tunnel walls and measured their "radioactive fingerprint." They found that the rock at their chosen spot was actually quite clean, with about 35% less radioactivity than rock found elsewhere in the tunnel. They also measured the air and found that while there is some radon gas (a radioactive gas that seeps out of rocks), it can be easily managed by pumping in fresh air, much like opening a window to clear out a smelly room.

The Vibration and Magnetic Hum
Finally, they listened to the ground and the air for invisible noise. They measured the magnetic field (the invisible force that guides compasses) and the seismic vibrations (the shaking of the ground). They found the magnetic noise to be incredibly low, except for a specific "hum" at 16.7 Hz. This hum turned out to be caused by the electric trains running in the nearby Furka railway tunnel; when the trains stop at night, the hum disappears. This tells the scientists that if they build their lab, they just need to shield against that specific frequency.

The seismic measurements were even more impressive. The ground at Bedretto is so still that it is far quieter than the "New Low Noise Model," which is the theoretical limit of how quiet the Earth can get. In fact, the vibrations are so low that they are two orders of magnitude better than what is required for next-generation gravitational wave detectors—machines designed to listen to the ripples in space-time caused by colliding black holes.

What This Means for the Future
The paper concludes that the Bedretto tunnel is a "highly competitive" location for fundamental physics. It's not just deep; it's accessible, with horizontal tunnels that make it easy to move heavy equipment in and out, unlike some labs that require digging vertical shafts. The team suggests that with standard construction—like adding a layer of concrete to the walls to block more radiation and neutrons—this site could become one of the deepest and quietest laboratories in Europe.

They aren't claiming to have built the lab yet, but they have proven the site is ready. The measurements show that the background noise is low enough to support cutting-edge experiments searching for dark matter, neutrinoless double beta decay, and gravitational waves. Essentially, they have found a perfect, quiet cave where the universe's secrets might finally be loud enough to hear.