The Great Pyramid of Giza, the largest and oldest in Egypt, was built for the Pharoah Khufu around 2560 BC.Researchers spent two years studying the pyramid using muonshigh-speed particles created by cosmic rays hitting Earth's atmosphere.Muons can penetrate deep into rock and reveal hidden voids and chambers to special detectors.Physicists used muons to detect a void at least 100 feet long above the Grand Gallery leading to Khufu's tomb.No one knows what the void is, but at least one archaeologist thinks it's not a room or chamber filled with artifacts.More than 4,500 years after their construction, the Pyramids of Giza continue to hide deep and ancient secrets.A team of more than three dozen researchers on Thursday announced the discovery of a huge and unexplored void in Khufu's pyramid, also called the Great Pyramidthe largest of three main structures in Giza.What's more, the team used cosmic rays to see through millions of tons of rock, locate the void, and estimate its size. The cavernous space is roughly 100 feet long and sits almost directly above the Grand Gallery: a towering passage that leads to the tomb of Pharoah Khufu."This void was hidden in the construction of the pyramid. It is not accessible and we needed this new technique, at the right time, to identify it and to discover it," Mehdi Tayoubi, a co-founder of the HIP Institute and a leader of the research effort, called ScanPyramids, told reporters during a Wednesday press briefing.The research group published its findings Thursday in the journal Nature."A lot of people tried to dig some tunnels looking for chambers," he added. "But as far as I know, no one has tried to dig something in this area. There was no theory expecting to find something as big as the Grand Gallery here."Tayoubi and his colleagues assert the "Big Void" is neither a data anomaly nor porous rock or loose rubble; in fact, the team is 99.9999% certain it exists, and a foremost expert on Egypt's pyramids agrees with that conclusion."I don't think this is bulls---," Mark Lehner, an archaeologist and Egyptologist at the Ancient Egypt Research Association, told Business Insider. "I put credence in the results, and I think they have indications of a large empty space."However, Lehnerwho served on an Egypt-based advisory committee overseeing ScanPyramids' project, yet wasn't involved in the worksaid the void is not likely packed full of undiscovered mummies, golden talismans, and other ancient artifacts.How to probe a pyramid with cosmic raysPyramids are confounding objects to study because they are so large; it's tough to see how they're constructed (and what they're hiding) without boring, digging, or blasting away their ancient and stony features."In the past it was easy. You have a scientist, and he says, 'I see a hidden door here,' and he can go and destroy this door and see what is behind it," Hany Helal, a pyramids expert at Cairo University and a ScanPyramids team member, told Business Insider during the press briefing. "Now, no one can allow for trial-and-error."Khufu's pyramid is a especially difficult case: It's not only the largest and most ancient Egyptian pyramid, making its hidden internal structure all the more labyrinthine, but also a prized national heirloom.Its construction of high and deeply embedded tunnels and chambers, like the Great Gallery, is also exceptional, Lehner said."In the popular imagination, it's the classic pyramid. But really it's the anomaly," Lehner added. "Nobody knew what Khufu did and no one did it afterward."However, the ScanPyramids team realized the same non-destructive technology used to peer inside volcanoes, called muography, might be used to probe structures like Khufu's pyramid.Muons are fast-moving, short-lived elementary particles that constantly shoot down from the sky. They're made when cosmic rays from supernovas, merging neutron stars, black holes, and other high-energy objects reach Earth and pummel air molecules.This creates a shower of particles, including muons, of which thousands pass through our bodies every minute.Muons pass through air unhindered, and get weakly absorbed or deflected by rock. So, by setting out special muon-detecting film and devices inside a pyramid, any cavities will show up as bright spots (since fewer are being absorbed by rock and more are making it to a piece of film or a detector).Inside the Great Pyramid, however, just 1% of muons reach inner chambers.This is why it took ScanPyramids two years to round up enough data: It takes months to expose a piece of film or run a detector, and the team used three different techniques to show beyond a doubt the void exists.Nothing to see here'With the void discovered, but some details about it unknown, including its exact height and angle within the pyramid The group is also wary about labeling it a "room" or "chamber," since it's not certain that is the case."For the moment it seems very difficult to access this very big void," Tayoubi said, adding that more research will be needed to define its shape and extent without destroying and internal masonry.Helal and others would ultimately like to bore a small hole into slabs of rock to reach the void, pop in a tiny yet capable robot, and explore the space remotely.Lehner is not sure anything other than secrets about the structure of the pyramid will be found inside, though.As he explained to Business Insider, Khufu's father determined what pyramids should look like, but it was under Khufu's reign that their construction was pioneeredand experimented with."Nobody is assessing the reality of, the fabric of the pyramid itself," Lehner said. "This pyramid looks very regular from a distance, but it was made with a considerable slop factor."For example, he said, there are many "relieving structures," including extra ceilings and spaces stacked above the King's Chamber near the pyramid's center. Lehner said ancient Egyptians over-engineered these sections as insurance policies against structural collapsea problem risked both by the crushing mass of the pyramid and shoddier construction of regions filled with rubble and sand.And that's where Lehner's self-described "shoot-from-the-hip" hypothesis about the void's purpose and role comes in."For all the world, I'll bet this is a relieving space for the Grand Gallery," he said, noting the feature exists right below the void. "They probably thought they needed some kind of relieving space to separate the slop factor from the fine masonry."Put another way: The big void may be nothing more than a stone-roof attic that protects a magnificent attraction several stories below it.ScanPyramids did not say how much its two-year effort cost in time for publication, but Lehner suspects it was expensiveand, despite commending the discovery, thinks such resources may be put to better use."Years ago I realized I had to turn my back to the pyramids to understand them," Lehner said. "To understand the pyramids, we need to understand how ancient Egypt workedtheir economy, where they lived, what they ate, what drove them."Lehner, who runs a field school for young archaeologists interested in ancient Egypt, added that the void's discovery is not really a "wow moment." More shocking, he said, was when archaeologists found a simple pyramid construction logbook.Such documents reveal much, much more about the pyramids than the monuments themselves, he added, and they're found only through traditional and relatively inexpensive archaeological field worknot bespoke particle physics devices."Would the people of a 3,000-year-old civilization be better served by finding a hole'" he asked. "Or a year's worth of Business Insider'"SEE ALSO:17 popular 'facts' about outer space that scientists have totally debunkedDON'T MISS:A student found an ancient Canadian village that's 10,000 years older than the PyramidsJoin the conversation about this storyNOW WATCH: This mysterious pyramid-shaped mountain in space is actually a gigantic ice volcano Click here to read full news..