The observatory, known as IceCube, detects several hundred neutrinos every day, but these are produced near Earth. On September 22, 2017, the instrument caught something unusual: a single neutrino unlike all the rest. It had far more energy than the others, which suggested it came from somewhere beyond the solar system.

This time, astronomers have figured out exactly where it came from.

An international team of astronomers have traced this single, high-energy cosmic neutrino to a supermassive black hole at the center of a galaxy nearly 4 billion light-years away. The discovery, announced Thursday and published in Science, marks the first-ever detection of the origin of cosmic neutrinos.

The story of this discovery begins billions of years ago, when the Earth was just beginning to take shape. In another corner of the universe, a gargantuan black hole churned at the heart of a sparkling galaxy. Black holes love to gobble up everything that comes near them, but they’re also quite good at spitting matter back out, creating high-energy streams that light up the darkness. The black hole spewed a jet of particles, including neutrinos, and sent them flying through space.

Skip ahead a little bit—to last year—and one of these neutrinos finally made it to Earth. It passed through the planet as it would anything else, but it didn’t go unnoticed. Scientists were ready for it.

They had drilled more than 80 narrow holes into the thick Antarctic ice, each stretching about 8,200 feet into the depths. Into the holes went more than 5,000 light sensors. Neutrinos, the scientists knew, are not governed by electromagnetic forces, but they can, on rare occasions, interact with the nucleus of an atom. This interaction generates a new kind of particle, and that particle produces a tiny flash of light the underground sensors can detect.

This is what happened in Antarctica. When the neutrino struck the ice and continued on its merry way, IceCube recorded the glint of its passing.

The IceCube astronomers sounded the alarm within minutes, galvanizing their colleagues around the world to help. The flash of light they detected gave them a sense of the direction the neutrino had come from. They broadcast these coordinates to their peers, who aimed telescopes at the relevant slice of the sky, scanning it in virtually every wave on the electromagnetic spectrum. Meanwhile, the IceCube team pored over archival telescope observations of the region. They found more than 600 potential candidates for the source of the neutrino.

After countless analyses were run, only one candidate was left standing: a special kind of galaxy called a blazar. Blazars house supermassive black holes that spray jets of high-energy particles pointed directly toward Earth. This one, known as TXS 0506+056, is located in the constellation Orion, visible in the night sky throughout the world.