The ancient Egyptian pyramids were built more than 4,000 years ago, and modern scientists have been studying them for two centuries. But as a research project illustrates, these iconic tombs are still teeming with mysteries.

Using infrared thermal scanning and other techniques, an international team of investigators first identified major anomalies in late 2015 at several of Egypt's most famous pyramids, including the Great Pyramid of Giza (aka the Pyramid of Khufu).

Now, the scientists are relaying the discovery of a huge, and heretofore unknown, space inside Giza. Their findings were released on Nov. 2 in a paper in the journal Nature. Its title: "Discovery of a big void in Khufu's Pyramid by observation of cosmic-ray muons."

Launched on Oct. 25, 2015, the #ScanPyramids project focuses on Khufu, Khafre, the Bent and the Red pyramids. The project combines several non-invasive and non-destructive scanning techniques "to detect the presence of any unknown internal structures and cavities in ancient monuments," according to a press release issued by Egypt's Ministry of Antiquities and the Heritage Innovation Preservation (HIP) Institute at the time.

Researchers initially performed thermal scans at the site at sunrise, when sunlight heats up the pyramids, and at sunset, when the structures begin cooling down again. If an object is solid — i.e., built with blocks of the same material that emit heat at similar rates — this shouldn't reveal any major temperature differences. On the other hand, if there are any quirks in the structure — like varying materials or hidden cavities — some parts will heat up or cool down more quickly than others.

Egypt pyramids infrared scanInfrared scanners are just part of the effort to unveil secrets in Egypt's pyramids. (Photo: Philippe Bourseiller/HIP)

"At the end of the first mission of #ScanPyramids, the teams ... have concluded the existence of several thermal anomalies that were observed on all monuments during the heating up or the cooling down phases," the Ministry of Antiquities says in a statement in late 2015

The latest anomaly to be unveiled, the "void," was detected by another type of scan, muon-tomography. Muons are kind-of electrons that form when rays from outer space collide with particles in our atmosphere. Scientists can measure the density of objects depending on the amount of muons present.

The "ScanPyramids Big Void" is at least 30 meters (96 feet) long. It's located just above the Grand Gallery — a long, narrow and steep passageway that leads to the King's Chamber — and is believed to be the first major interior find in Giza since the 19th century. "While there is currently no information about the role of this void," the paper's authors write, "these findings show how modern particle physics can shed new light on the world's archaeological heritage."

That method might be the big breakthrough of the project, especially considering some archaeologists already are downplaying the findings themselves.

From the New York Times: "Many archaeologists questioned whether the study offered any new information about the ancient Egyptians, and were quick to note that the team most likely did not find a hidden room filled with the pharaoh’s riches. They said the so-called void was probably empty space designed by the pyramid’s architects to lessen the weight on its chambers and prevent them from collapsing, an example of features that were already documented in the construction of the ancient monuments."

Still, nobody knows for sure, and the method the researchers have employed may show the way to someday find exactly what is in the space — if anything.

"To our knowledge," the authors write in Nature, "this is the first time an instrument has detected a deep void from outside a pyramid."

Opening the door to more discoveries

Because the ScanPyramid project has been ongoing, this is not the first discovery at the tomb of the pharaoh known by the Greeks as "Cheops." Thermal imaging turned up some findings early on in the project.

Thermal scans showed that the pyramid's first row of limestone blocks all have roughly the same temperature, according to Antiquities Minister Mamdouh Eldamaty, aside from three that are also "different in formation" from other blocks. And while inspecting the ground in front of the pyramid's eastern side, Eldamaty says the researchers also found "there is something like a small passage leading up to the pyramid ground, reaching an area with a different temperature."

Egypt pyramids infrared scanA thermal anomaly on these blocks has raised new questions about the ancient pyramids. (Photo: Philippe Bourseiller/HIP)

No one is sure yet what any of the anomalies mean — they may hint at gaps or fissures in the walls, carefully planned spaces or hidden passageways or chambers.

This teaser video, released by the Egyptian Ministry of Antiquities and the Paris-based HIP Institute, offers more detail about the project:

"In the longer term, given the archaeological wealth of Egypt, we imagine applying these techniques to other monuments," Cairo University professor and project coordinator Hany Helal said in a statement back in 2015. "Either to restore or to discover them. If these technologies are effective, they can even be implemented in other countries."

Editor's note: This article first appeared in November 2015. It has been updated with new information.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.