Unable to dig, researchers look to cosmic rays to unlock Jerusalem’s ancient underworld

A Tel Aviv University team is using muon detectors to track powerful particles, hoping to build a 3D map of undiscovered tunnels, chambers and fortifications under the hole-y city

Reporter at The Times of Israel

Researchers build a muon detector in a cave under the City of David in 2023. (Courtesy: Gilad Mizrachi / Tel Aviv University)
Researchers build a muon detector in a cave under the City of David in 2023. (Courtesy: Gilad Mizrachi / Tel Aviv University)

Last year, a team of Tel Aviv University physicists and archaeologists wriggled and rappelled their way into an underground cavern outside the Old City of Jerusalem, using ropes and pulleys to lug down a sizable contraption.

The large gizmo, consisting of stacks of metal components linked by tangles of wires, tubes, ribbons and clamps, was about the least likely thing one would expect to find in the rocky recesses of the cave, like a time machine plopped 3,000 years in the past.

In some ways, that’s exactly what it was.

Known as a muon detector, the device had been situated in the cavern in hopes of using subatomic particles to image heretofore undiscovered secret passageways leading to the Gihon Spring, the city’s source of water in ancient times — without picking up a shovel.

“All ancient cities needed a water supply and fortified, secret ways to get to there in case of emergency or war,” said Prof. Oded Lipschits, director of Ancient Israel Studies at the university and one of four coordinators of a project to map the tunnels, chambers, and canals underneath Jerusalem.

The device, which had been built by the Tel Aviv University team, was designed to capture and register the angular distribution of muons, tiny but powerful particles created when cosmic rays smash into the earth’s atmosphere.

Researchers move a muon detector into a cave under the City of David in 2023. (Courtesy: Gilad Mizrachi / Tel Aviv University)

For archaeologists, using particle physics to peer into solid ground offers a tantalizing way to glimpse the history of a city densely packed with strata upon strata of ancient settlement, but just as chock full of religious sensitivities. This makes invasive digs under many of its most important sites — especially the Temple Mount — a fraught, if not impossible, endeavor.

According to Lipschits, the method, known as muon radiography, could shed light on life in Jerusalem long ago, including the fortifications built by the city’s denizens, the tombs and spaces they might have carved, and their access to water during enemy sieges.

Researchers believe the detector will leave no stone unturned — without turning over any stones, or disrupting people and buildings above ground.

The Gihon Spring at Jerusalem’s City of David. (Tal Glick/City of David Facebook)

“It is our dream to scan underground in a search for tunnels that might lead us to the Temple Mount,” said Prof. Erez Etzion of the School of Physics and Astronomy. “The muon detectors can provide us with tools to search underground passively.”

For the project, Etzion, Liron Barak and a team from the physics school were working together with Prof. Yuval Gadot of the Rosenberg School of Jewish Studies and Archaeology, as well as with Lipschits, Yiftah Shalev from the Israel Antiquities Authorities and a team from Rafael Industries, including Yiftah Silver.

Aside from the Gihon Spring, a second muon detector had been set up behind what is known as the Stone Stepped Structure, an enormous wall in the City of David dating to some 3,000 years ago.

A view of the modern neighborhood of Silwan, showing the eastern ridge where the ancient City of David was located. (iStock/Klug-Photo)

The spot, about halfway between the Gihon Spring and the Temple Mount, is the deepest point on the eastern slope of the City of David, a narrow ridge that descends from just south of the Temple Mount toward the Kidron Valley, outside the Old City walls and home to the modern neighborhood of Silwan. Archaeologists have identified the area as the oldest part of Jerusalem, linking it to biblical accounts of the city.

By pointing the detector toward the Temple Mount, the researchers hope to use it to discover tunnels or underground spaces running from beneath the undiggable platform toward the spring.

Lipschits and Etzion have known each other since high school in Kiryat Tivon, where they played together on the water polo team more than forty years ago. “Ancient history,” quipped Etzion.

At the university, the two professors talked about different ideas they were working on and then decided to team up on this project that combines physics and archaeology.

Erez Etzion, seated, and Oded Lipschits. (Courtesy: Gilad Mizrachi / Tel Aviv University)

“As a historian, I believe it’s important to understand our history,” Lipschits said. “The City of David is the most important site for all three of the world’s monotheistic religions.”

Muons are emitted in the upper atmosphere by cosmic rays from outer space which interact with atoms in the Earth’s atmosphere. The muons travel nearly at the speed of light and are almost unstoppable, capable of penetrating just about anything, including solid rock. By setting up the detector at a lower elevation than the target area and angling it correctly, the scientists can analyze the rate of the muons passing through whatever they wish to study.

“As the muons are coming from the sky, you need to place the detector in a spot that is lower than the screened region, to see if there is any deviation in the ground between that detector and the surface,” explained Etzion.

If the detector receives more muons than expected, it means that, for at least part of their underground journey, the particles didn’t travel through matter but through an empty cavity, Etzion said.

A model showing the City of David as it may have looked during the Second Temple period. The Siloam pool, fed by the Gihon Spring, can be seen toward the bottom. (iStock/Linda.Johnsonbaugh)

The scientists’ research is based on “an old idea” from the 1960s, when Nobel-winning physicist Luis Alvarez’s team of researchers used muon detectors to scan inside the Pyramid of Khafre in Egypt.

Alvarez didn’t “find the secret thrones,” but “he did come up with a workable device,” Etzion said. In 2023, also using muon detectors, researchers discovered an unknown chamber in the Great Pyramid in Giza.

Scientists in Italy are now working to use muons to map the interior of Mount Vesuvius, to model the eruptive dynamics of the volcano that destroyed Pompeii in 79 CE and still poses a threat to the city of Naples.

Egyptian Antiquities Minister, Khaled El-Anani, left, and Zahi Hawass, Egypt’s former head of antiquities, second left, listen to French expert Sebastien Procureur in front of a muon detector at a tent in front of the Great pyramid, in Giza, Egypt, Thursday, June 2, 2016. (AP/Amr Nabil)

In Jerusalem, the researchers are hoping to set up a whole array of detectors within the next two years. With multiple detectors, using what is known as muon tomography, the team believes it can not only spy where cavities are, but also get readings on their dimensions and what may be inside, building a 3D map of the ancient city’s underground.

The physicists are currently working on developing an innovative technique that increases the detector’s precision and ability to detect smaller voids.

“You can’t go out and buy a muon detector,” Etzion said wryly. “You have to build it yourself.”

Etzion was involved in building muon detectors for the ATLAS experiment at CERN Large Hadron Collider, in Switzerland. However, he said that was “installing a detector in a laboratory setting, which is far different” from constructing a detector underground.

“It was an extremely challenging task,” Etzion recalled. “In the lab, we are in a sterile environment; here, we rebuilt a detector in a few crazy days, under the ground where it’s dark and humid, far away from a clean laboratory with ideal conditions.”

Researchers placing a muon detector in a cavern beneath the Gihon Spring in 2023. (Courtesy: Gilad Mizrachi / Tel Aviv University)

Adding to the challenge, the detectors also have to remain in the dank underground environment for extended periods. The rate of muons penetrating the ground is very slow, and collecting data currently takes months, Etzion said. Nonetheless, he found the detector works “surprisingly well.”

“It really surprised us that it worked at all,” he admitted.

Beyond archaeology, the method could be employed to help detect tunnels being dug by the Hamas and Hezbollah terror groups under the Gaza and Lebanon borders respectively. For now, though, the Tel Aviv University team is keeping its detectors trained on tunnels of a more ancient vintage.

“We’re continuously improving the design of the devices enabling operation and detection in harsh underground conditions,” Etzion said. “We have demonstrated that we can provide a real mapping of hidden tunnels and canals of ancient Jerusalem.”

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