Hunting Ghost Particles Beneath the World’s Deepest Lake

The lake is as much as a mile deep, with some of the clearest fresh water in the world, and a czarist-era railroad conveniently skirts the southern shore. Most important, it is covered by a three-foot-thick sheet of ice in the winter: nature’s ideal platform for installing an underwater photomultiplier array.

“It’s as if Baikal is made for this type of research,” said Bair Shaybonov, a researcher on the project.

Construction began in 2015, and a first phase encompassing 2,304 light-detecting orbs suspended in the depths is scheduled to be completed by the time the ice melts in April. (The orbs remain suspended in the water year-round, watching for neutrinos and sending data to the scientists’ lakeshore base by underwater cable.) The telescope has been collecting data for years, but Russia’s minister of science, Valery N. Falkov, plunged a chain saw into the ice as part of a made-for-television opening ceremony this month.

The Baikal telescope looks down, through the entire planet, out the other side, toward the center of our galaxy and beyond, essentially using Earth as a giant sieve. For the most part, larger particles hitting the opposite side of the planet eventually collide with atoms. But almost all neutrinos — 100 billion of which pass through your fingertip every second — continue, essentially, on a straight line.

Yet when a neutrino, exceedingly rarely, hits an atomic nucleus in the water, it produces a cone of blue light called Cherenkov radiation. The effect was discovered by the Soviet physicist Pavel A. Cherenkov, one of Dr. Domogatski’s former colleagues down the hall at his institute in Moscow.

If you spend years monitoring a billion tons of deep water for unimaginably tiny flashes of Cherenkov light, many physicists believe, you will eventually find neutrinos that can be traced back to cosmic conflagrations that emitted them billions of light-years away.

The orientation of the blue cones even reveals the precise direction from which the neutrinos that caused them came. By not having an electrical charge, neutrinos are not affected by interstellar and intergalactic magnetic fields and other influences that scramble the paths of other types of cosmic particles, such as protons and electrons. Neutrinos go as straight through the universe as Einsteinian gravity will allow.