At the South Pole, the ice is almost 3 kilometres thick. Deep inside it, in total darkness, thousands of glass spheres the size of basketballs hang on long cables. They’re arranged in a giant grid that fills a cubic kilometre of ice, a block large enough to hold several hundred Great Pyramids.
This is the IceCube Neutrino Observatory. It’s one of the strangest telescopes ever built, because it doesn’t look at the sky. It watches the ice, waiting for faint flashes of light caused by ghostly particles from deep space.
The particles it’s hunting
IceCube is designed to detect neutrinos. They are tiny, nearly massive-less particles produced in nuclear reactions, from the Sun’s core to exploding stars and violent galaxies.
Neutrinos are extraordinarily hard to catch. They have no electric charge and barely interact with anything. Trillions pass through your body every second without leaving a trace. Most could pass through the entire Earth without hitting a single atom.
That’s exactly why astronomers want them. Light from distant objects can be blocked by dust or absorbed along the way, but neutrinos travel in straight lines from where they were made. They carry information from places that ordinary telescopes can’t see.
How ice becomes a detector
Very rarely, a neutrino does hit an atom in the ice. The collision produces a charged particle that moves faster than light can travel through ice. That creates a brief flash of blue light called Cherenkov radiation, a little like an optical version of a sonic boom.
The ice deep below the South Pole is among the clearest natural material on Earth, so that light can travel a long way. IceCube’s sensors record when and where the flash arrives. From those patterns, scientists can work out the neutrino’s energy and the direction it came from.
Building it in the coldest place on Earth
Construction ran from 2004 to 2010. Engineers used a high-powered hot water drill to melt 86 holes, each about 2.5 kilometres deep. Into each hole, they lowered a cable holding 60 sensors before the water refroze around it.
The original detector had 5,160 sensors, placed between about 1,450 and 2,450 metres below the surface. Work could only happen during the short Antarctic summer, when temperatures rise to around minus 30°C.
In the summer of 2025 to 2026, IceCube was expanded for the first time. A project called the IceCube Upgrade added more than 600 new and improved sensors and calibration devices in a dense cluster at the center of the array.
Looking through the whole planet
One of IceCube’s cleverest tricks is to use the Earth itself as a shield. The detector is constantly hit by particles called muons, created when cosmic rays strike the atmosphere above Antarctica. These can drown out the far rarer neutrino signals.
Muons from above can’t pass through the whole planet. But neutrinos can. So when IceCube sees a particle traveling upward through the ice, it knows it was almost certainly a neutrino that entered the Earth on the other side, often from the northern sky.
In effect, the South Pole telescope looks down, through about 12,700 kilometres of rock, metal and ocean, at the sky above the Arctic.
What it has found
In 2013, IceCube announced the first clear detection of high-energy neutrinos from beyond our solar system. Some of those particles carried more energy than any human-made particle accelerator can produce.
In 2017, it detected a neutrino traced to a blazar, a distant galaxy whose giant black hole fires a jet toward Earth. In 2022, it linked neutrinos to the nearby active galaxy NGC 1068. And in 2023, it produced the first image of the Milky Way in neutrinos instead of light.
When IceCube spots an unusually energetic neutrino, it sends an automatic alert to observatories around the world within minutes. Telescopes can then turn toward that part of the sky to look for a matching flash.
What comes next
The new sensors from the Upgrade will help scientists understand the ice more precisely and improve measurements of the neutrinos themselves. That should sharpen even the detector’s older data, which can now be reanalysed.
Scientists have also proposed IceCube-Gen2, a much larger observatory that would monitor about eight times the volume of ice. If it’s built, the South Pole will keep its unusual role as one of the best places on Earth to watch the universe, by looking down instead of up.