In December 2015, a group of scientists carried tools into a chamber inside of the Great Pyramid of Giza. Usually, the room was sealed from the public. But with the blessing of Egypt’s Ministry of Antiquities, they used laser tools to carefully align several bathroom-tile sized panels on the floor of the last intact Wonder of the Ancient World. Each panel contained a special photographic film.

They left the panels there for more than three months. If all went as planned, the panels would capture images they could use to find new chambers and passageways in the pyramid. The pyramid’s known rooms include the queen’s chamber—where they installed the panels—the king’s chamber with its looted sarcophagus, and a sloping, high-ceilinged room known as the Grand Gallery. But the possibility remained that more treasures lay hidden in the 4,500-year-old, 50-story structure.

The group announced a discovery on Thursday. Publishing in Nature, the team of researchers from Egypt, France, and Japan, chronicle a new space, as long as the Statue of Liberty, above the Grand Gallery. Because they don’t know the intended purpose of the space, they won’t call it a “chamber,” preferring to call it a “void.” “The void is there,” said Mehdi Tayoubi, the president of the Heritage Innovation Preservation Institute, during a press conference. “What is it? We don’t know.”

For one thing, the group isn’t made up of traditional mummy-curating Egypt experts. In addition to his work at HIP Institute, Tayoubi is an executive at a 3-D design software company in France. The team actually includes a large number of physicists—because those special photographic films they left in the queen’s chamber actually have a lot in common with experiments at the Large Hadron Collider.

The panels are known as nuclear emulsion films, designed to record pictures of tiny elementary particles called muons. Muons are negatively charged like electrons, but about 200 times heavier. They form when cosmic rays—extremely high energy particles flying toward Earth from outer space—interact with atoms in the atmosphere. “One muon passes through your hand per second,” says physicist Paolo Checchia of INFN Padova, who is unaffiliated with the project. Physicists also make muons at the LHC, when the collider smashes protons together at high energy. In fact, that’s one of several ways that physicists discovered the Higgs boson in 2012—not by seeing the Higgs itself, but the muons it turned into. And they did it with a muon detector that Checchia helped build.

Over three months, millions of muons sprinkled down on the Great Pyramid—and tunneled right through the Grand Gallery, onto the emulsion panels, and further down the pyramid. Muons can shoot through half a mile of rock. But solid material does alter the particle’s path, which means you can track a muon to create an image of the brick it just flew out of. “It’s the same principle as X-rays in the hospital,” says physicist Jacques Marteau of the Institute of Nuclear Physics of Lyon. But muons can see much deeper than X-rays. Marteau has actually used muons to look inside volcanoes to observe the magma levels inside.

The film, whose design efforts were led by physicist Kunihiro Morishima of Nagoya University, consisted of a thin layer of silver bromide, the same chemical on traditional photography film. When a muon flew through the film, the chemical would react, marking the muon’s path. “The analogy is, it's like tracking a jet by its contrail, and not the airplane itself,” says physicist Roy Schwitters of the University of Texas at Austin, who is using muons to look inside Mayan pyramids in Belize. Using a computer, Morishima’s team could calculate the path of the muons and infer how much material they’d traveled through. Muons on certain parts of the film, they found, had traveled through less material—and voilà, evidence for a vast, unexplored void.

Which is funny, because emulsion films have long been used to study the vastest void of all. Before anyone placed them in the Great Pyramid, physicists were using them to study the smallest building blocks in our universe. In 1947, they discovered a new particle called a pion with the films. They still use the emulsion films in particle experiments: On other projects, Morishima is using them to study cosmic rays and neutrinos, a type of particle even more penetrating than a muon.