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A previously hidden corridor buried deep in the 4500-year-old Great Pyramid of Giza in Egypt has been mapped in detail for the first time – and researchers have also taken a glimpse inside using an endoscopic camera.

The corridor was first discovered in 2016, but researchers didn’t want to damage the monument to gain access. The pyramid is the only one of the seven wonders of the ancient world still standing, and was, for millennia, the world’s tallest human-made structure at 146 metres. It was built around 2560 BC during the reign of the pharaoh Khufu.

Using a technique called cosmic-ray muon radiography developed by academics at Nagoya University, Japan, an international team of researchers was able to confirm that the corridor was 9 metres long, with a cross section of about 2 metres by 2 metres.

Cosmic-ray muon radiography tracks the level of muons passing through the pyramid. These particles are a natural form of radiation resulting from cosmic rays and are constantly bombarding Earth’s surface. In the technique, researchers use muon detectors placed at various points around the monument. Muons are partially absorbed by the stone used to build the pyramids, which means the method allows researchers to identify cavities inside the structure.

This approach has been used to map the internal structures of pyramids , when it was first used at Giza.

Using their precise map of the corridor, the researchers identified an opportunity. “We realised that it was so close to the surface that an endoscopy was possible,” says at the University of Paris-Saclay in France.

They inserted a small camera similar to those used in medical procedures to get the first glimpse of the corridor in thousands of years.

“We knew the cavity was there, but of course it’s totally different when you see it,” says Procureur. “We felt strange when we saw this.”

Still, Procureur was glad of one thing. “It’s a controversial opinion, but I’m relieved the cavity was empty. I wouldn’t have liked to participate in opening a tomb.”