Archaeologists have discovered a huge, unidentified space deep inside the Great Pyramid of Giza, the massive ancient structure built on the outskirts of Cairo some 4,500 years ago.

The international team of scientists has been using a sophisticated imaging technique based on cosmic radiation to peer inside the pyramid and map its internal structure.

Now, after two years of study, they have identified a mysterious void hovering just above the pyramid’s impressive Grand Gallery — the sloping cathedral-like passageway at the center of the pyramid that links the Queen’s burial chamber to the King’s above it.

Researchers believe the newly discovered space has similar dimensions to the gallery, which measures almost 50 meters (164 feet) long and 8 meters (26 feet) high.

But they are not sure whether it’s simply a structural feature designed to relieve weight on the gallery or whether it serves some other — possibly more important — purpose.

The 4th Dynasty pharaoh Khufu, or Cheops, had the Great Pyramid constructed during his reign, which lasted from 2509 to 2483 B.C.

Excavations beginning in the late 1800s revealed three chambers within the 140-meter (459-foot) high pyramid: a subterranean chamber at the base, the Queen’s Chamber in the middle and the King’s Chamber above.

No mummies have ever been found inside the Great Pyramid, leading some to speculate that it may not even have been intended as a tomb.

But most Egyptologists believe that looters removed Khufu’s mummy, along with the riches presumably buried with the pharaoh, before the excavations took place.

As part of the ScanPyramids project, launched in 2015, Japanese and French scientists have been scanning the Great Pyramid using a technique called muography.

Muons, or subatomic particles of cosmic radiation, make their way to Earth when cosmic rays collide with atoms in the upper atmosphere.

They act effectively like x-rays when they hit, allowing scientists to see the inner structure of monuments without drilling inside or otherwise damaging them.

While the stone used to build the pyramid absorbs the muons, chambers and other cavities allow them to pass through.

According to their findings, published this week in Nature, the scientists knew they had stumbled on something big when they located a hotspot of muons above the gallery.

“We don’t whether this big void is horizontal or inclined; we don’t know if this void is made by one structure or several successive structures,” Mehdi Tayoubi, from the HIP Institute, told BBC News.

“What we are sure about is that this big void is there; that it is impressive; and that it was not expected as far as I know by any sort of theory.”

The ScanPyramids researchers are now calling on experts in ancient Egyptian architecture to speculate on what the enclosure might be.

They don’t plan to drill inside, but are constructing a tiny robot they will send in to investigate, provided the Egyptian government gives them the go-ahead.

Though scientists have been studying the pyramids for hundreds of years, we still know relatively little about why they were built, how they were built and what they were used for.

As the first major feature discovered in the Great Pyramid since the 19th century, the mysterious void could provide new clues into these enduring questions.

By Sarah Pruitt, Guest writer