For 4,500 years, the Great Pyramid of Giza has guarded its secrets behind millions of tonnes of limestone. We have mapped its corridors, crawled its shafts, and stood inside the eerie, soaring chamber of the Grand Gallery. We assumed we knew the inside of the most studied building on Earth.

Then, in 2017, physicists pointed a stream of subatomic particles at it — and the pyramid gave up a secret it had kept since the age of the pharaohs. High above the Grand Gallery, sealed in solid stone, lies a hidden space at least 30 metres long. No door leads to it. No text describes it. And to this day, no one can say for certain what it is.

A pyramid scanned with cosmic rays

The discovery did not come from a shovel or a drill. It came from outer space.

When cosmic rays from deep in the galaxy strike Earth’s upper atmosphere, they shatter into showers of particles called muons. These muons rain down on us constantly — they are passing through your body as you read this. Crucially, muons are absorbed and slowed more by dense rock than by empty air. So if you place sensitive detectors beneath or beside a massive stone structure and count the muons that make it through, the empty spaces inside reveal themselves as bright patches where more particles get through. It is, in effect, a way to take an X-ray of a mountain of stone without touching a single block.

This is exactly what the international ScanPyramids mission set out to do. Beginning in 2015, teams from Egypt, France, Japan and elsewhere installed muon detectors inside and around the Great Pyramid of Khufu, alongside infrared thermography and other non-invasive tools. The pyramid is far too important — and far too sacred — to be tunnelled into on a hunch. Muons offered a way to look inside without harming the monument at all.

The “Big Void” above the Grand Gallery

In November 2017, the team published their headline result in the journal Nature, and it stopped Egyptologists in their tracks.

Three different muon detection technologies, deployed independently, all pointed to the same thing: a large, previously unknown cavity sitting directly above the Grand Gallery — the steep, vaulted passage that climbs toward the King’s Chamber. The researchers called it simply the “Big Void.” Its cross-section appears similar to that of the Grand Gallery itself, and it has a minimum length of around 30 metres — roughly the length of a basketball court hidden inside a 4,500-year-old building.

This was no minor footnote. It was, the team noted, the first major internal structure discovered in the Great Pyramid since the 19th century. For more than a hundred years, the pyramid had seemingly given up all it had to give. Cosmic particles proved it had been hiding something enormous all along.

The most studied building in the world had kept a secret the size of a cathedral, and we only found it because particles from a dying star happened to pass through.

A second, smaller secret on the north face

The Big Void was not the only thing ScanPyramids found. On the pyramid’s north face, behind the great chevron-shaped stones above the original entrance, the muons hinted at another anomaly — a smaller corridor.

This one, unlike the Big Void, could be reached. In 2023, researchers located a tiny gap between the stones and threaded through a state-of-the-art endoscope just five millimetres wide. For the first time, a camera looked inside a space no human had seen since the pyramid was sealed. The images revealed a corridor with a vaulted ceiling, roughly nine metres long and two metres wide, now formally named the ScanPyramids North Face Corridor. Its existence was further confirmed in 2025 in the journal Scientific Reports, using a fusion of muon imaging, ground-penetrating radar and ultrasound.

The corridor itself was empty of artefacts. But its likely purpose is telling: many archaeologists believe it was built to relieve some of the immense weight pressing down on the structure below, perhaps around the original entrance — a piece of hidden ancient engineering, revealed at last.

So what is the Big Void?

Here is where honesty matters more than spectacle: nobody knows for sure. No one has yet reached the Big Void, and until someone does, every explanation remains a hypothesis. But a few serious ideas are on the table.

The first, and perhaps most likely, is structural. The builders of the Great Pyramid were master engineers, and they knew that the open span of the Grand Gallery created tremendous stress in the stone above it. Just as they stacked five “relieving chambers” above the King’s Chamber to spread its load, the Big Void may be an empty space deliberately left to take pressure off the gallery below — a hidden room whose job was simply to hold the weight of a mountain.

A second idea, proposed by archaeologist Kate Spence of the University of Cambridge, is that the void is a leftover of how the pyramid was actually built — possibly the trace of an internal ramp used to haul enormous roofing blocks into place, later abandoned or loosely filled with rubble. If so, the void would be less a chamber than a fossil of the construction process itself, a clue to the still-debated question of how the pyramids were raised at all.

And then there is the possibility that captures every imagination: that it is a hidden chamber. Khufu’s mummy has never been found. The granite sarcophagus in his King’s Chamber was discovered empty. It is tempting — almost irresistible — to wonder whether a sealed, undisturbed room above the Grand Gallery might hold something never seen by modern eyes. Most experts urge caution here. There is no evidence of a burial inside the void, and the structural explanation is far more conservative. But the question lingers, because the pyramid has earned its mystery.

Why we may never simply walk in

If the void is so tantalising, why not drill a small hole and send in a camera, as was done on the north face?

The answer is respect. The Great Pyramid is one of humanity’s most precious monuments — the last of the Seven Wonders of the Ancient World still standing. Any intervention risks damaging an irreplaceable structure, and Egyptian authorities are rightly protective. The whole point of the muon method was to look without touching. Future missions may eventually find a natural gap, as they did with the corridor, or refine their scans to map the void in finer detail. But there is no guarantee anyone will set foot — or even a lens — inside the Big Void in our lifetimes. It may keep its secret a while longer.

Why it grips us

There is something profoundly humbling about this discovery. We tend to think of the ancient world as fully explored, its monuments measured and mapped and demystified. The Big Void is a reminder that this is not true — that even inside the most famous structure ever built, vast unknowns can hide in plain sight, waiting for a tool no pharaoh could have imagined.

It also collapses an astonishing span of time and science into a single moment. Particles born in cosmic violence, falling for millions of years, slipped through stone laid by hand 4,500 years ago and whispered that something was hidden there. The builders are long gone. Their secret is not.

So here is the thought to sit with: if the most examined building on Earth could conceal a space the size of a great hall for four and a half thousand years, what else might still be waiting — sealed, silent — just beyond the reach of our oldest questions?