Neutrinos From Deep Inside Earth Provide a New Picture of the Mantle

In a laboratory 2 kilometers underground, a crane lowers Matt Depatie, a detector technologist, through a hatch into a white-walled cavern filled with about 7,000 tons of ultrapure water that glows as blue as wiper fluid in the light. Normally, the cavern is one of the darkest places on Earth, but today, it is lit up for maintenance, offering us a rare chance to see inside. I peer through the hatch at Depatie as he paddles over to examine the submerged experiment.

He’s inspecting a house-size detector built to catch some of the most elusive particles known to physics: neutrinos. This is the SNO+ neutrino experiment, buried deep within the Creighton mine at Snolab, an underground physics laboratory in Sudbury, Canada. SNO+ consists of an acrylic sphere lined with nearly 10,000 sensitive light detectors and filled with 780 tons of oily liquid scintillator, which flashes when lit up by energetic particles.

The water around the device and the rock above it shield the detector from the glare of cosmic radiation, allowing the flickers of less common particle interactions to shine through. Our whole journey down has been part of the crusade to maintain absolute darkness, even beyond the visible spectrum of light. As we descended the main shaft and walked through a rocky tunnel to the lab, our bodies and clothes collected minute amounts of radioactive radon dust.

Before entering the main laboratory space, we tossed our mine clothes, showered, and changed into electric blue jumpsuits and hairnets to minimize the contamination we carried in with us.