Deep in the Swiss Alps, there is a vertical shaft that plunges 800 meters down, like two skyscrapers stacked on top of each other. A railway tunnel runs nearby, and trains thunder through it every day. But inside the shaft, it is as quiet as a library at night.
Scientists brought sensitive sensors there and spent a month listening to how the ground trembles and what is happening with electric fields. Even when a train races past, the vibrations remain tiny — smaller than the footsteps of a person in the next room. The electromagnetic background also turned out to be very weak, as if the shaft shields everything extraneous.
If such an instrument were built in this shaft, it could search for gravitational waves in a range that no one has heard yet. It could also pick up traces of ultralight dark matter particles that might be passing through us every second. Then this quiet hole in the mountain would become an ear pressed against the very fabric of the universe.
The shaft is 800 meters deep — about the height of the world's tallest building, the Burj Khalifa, but pointing downward. And the deeper you go, the less surface vibrations interfere.