Great Pyramid of Giza yields another secret: a large hidden cavity

The Great Pyramid of Giza has been one of the world's most enduring man-made mysteries. For centuries, scientists and archaeologists have probed inside the largest and oldest of Egypt's famed pyramids.

Now, an international team of scientists has discovered a large hidden cavity within the pyramid Giza. And they did it in a fittingly 'unearthly' manner - by looking for muons, particles sent to earth by cosmic rays from space.

The mysterious cavity, described Thursday in the journal Nature, is at least 30 meters long. And though the researchers aren't sure whether it's straight or inclined, whether it's one large space or a series of smaller ones, the discovery has already triggered interest among archaeologists as to the purpose of the void.

Researchers hope the discovery will help them understand more fully how the massive pyramid was built.

The Great Pyramid contains three previously discovered rooms: an underground chamber, the King's chamber and a smaller Queen's chamber, connected by corridors and a passageway called the Grand Gallery. But this marks the first discovery of a major space inside the ancient structure since the 19th century, the journal says.

''What we are sure about is that this big void is there,'' said Mehdi Tayoubi, president of the nonprofit Heritage Innovation Preservation Institute in Paris, which led the effort. ''But we need to understand [it] better.''

The Great Pyramid of Giza was built during the rule of the Pharaoh Khufu, also known as Cheops, whose reign lasted from 2509 to 2483 BC. The largest of the pyramids, it stands some 139 meters high and 230 meters wide. Tourists enter the pyramid from a tunnel that was dug on the orders of Caliph al-Ma'mun around AD 820.

The Grand Gallery, which measures 8.6 meters high, 46.7 meters long and up to 2.1 meters wide. The King's and Queen's chambers each have two ''air shafts'' that were mapped by robots between 1990 and 2010.

But much of the pyramid's interior structure has remained a mystery, in part because there are very few documents from Khufu's time describing the building's design and construction.

Other research teams have searched for hidden ''chambers'' by measuring tiny variations in the pyramid's gravity or by using ground-penetrating radar. The results of those efforts have been inconclusive.

Using muons
For this project, a team of researchers attempted to boost their odds of success by adding muons to their scientific toolkit.

Muons are subatomic particles that are produced when cosmic rays - high-speed atom fragments that hurtle through space - smash into the atmosphere. They carpet-bomb earth at a rate of 10,000 per square meter per minute.

These tiny particles can pass through hundreds of meters of rock before decaying or being absorbed. This turns out to be very handy for scientists who want to probe a pyramid somewhat in the way that doctors use X-rays to examine a bone inside your body.

This technique had been pioneered decades ago by Nobel laureate Luis Alvarez of University of California Berkeley, who used muon tomography to probe the smaller Khafre pyramid nearby. He concluded that there were no large hidden voids within the structure.

Scientists with the ScanPyramids project who are studying the Great Pyramid had already found hints of what appeared to be a corridor behind the chevrons on the structure's north face.

The researchers realized they were on to something after they placed their detectors inside the Queen's chamber of the pyramid and found an unexpectedly large number of muons passing through, suggesting there was a previously unidentified "void," or empty space, above the chamber.

Three teams of physicists repeated the measurements to be sure of their finding. Each time, the conclusion was the same.

The large space could be horizontal, or sloped like the Grand Gallery. Its purpose, and what may lie inside, is still unclear. Scientists currently have no plans to use an intrusive technique to find out.

"It's the Great Pyramid, we can't touch it," said Mohamed Ismail, a spokesman for the Egyptian Ministry of Antiquities. "Any intrusive technique is out of the question."

Instead, the ministry wants other researchers to come forward with proposals to use different or more advanced techniques to verify the finding.

The banning of intrusive techniques, such as drilling holes through the stone and inserting a camera, will prevent damage to the pyramid, which sits 139 meters (about 455 feet) high and was built by Pharaoh Khufu some 4,500 years ago. It's the largest of the group of pyramids at Giza, one of the Seven Wonders of the Ancient World and a major tourist attraction.

Ismail hopes the published research will spark a scientific debate about why the void exists and what might be inside. But he dismisses any theories that the space contains another grand gallery or a burial chamber.

"There is no scientific, archaeological or historical evidence that corroborates this," he said. "If there is another burial chamber, there would have been an entrance to it."