ome of the ancient history underneath Jerusalem is easily accessible.
Pilgrims and tourists can get their feet wet in the Siloam Tunnel, carved by order of King Hezekiah to bring water inside the walls of the city (2 Kings 20:20). The Western Wall Tunnel, excavated in the late 20th century, traces a first-century street from the Western Wall to the path that Jesus is traditionally thought to have walked to his crucifixion. Archaeologists are currently excavating another street that pilgrims used in Jesus’ day to go from the Pool of Siloam (John 9:7) up to the Temple Mount.
But other parts of the city’s 5,000-year history are harder to get to—if not impossible. Practical and political problems prohibit access, even for the experts trying to do research.
Now, however, physicists have come up with a new way to dig without digging: muography.
Muons are tiny subatomic particles that are everywhere on earth, according to physicists. They are created when cosmic rays smash into the Earth’s atmosphere, showering the surface of the planet with about 10,000 of the particles per square meter.
In recent years, scientists have figured out how to use muon detectors to map inaccessible subterranean cavities, creating images of rooms inside Egyptian pyramids and magma chambers deep in volcanoes. Now they’re using them to map the streets Jesus once walked in ancient Jerusalem.
Last year, a team of Tel Aviv University archaeologists and physicists shoehorned an unwieldy homemade muon detector—you can’t buy one from a store—into a rocky cavern close to the Gihon Spring in the Kidron Valley. They placed another detector behind a rocky bulwark called the Stepped Stone Structure. Then they pointed them both toward the Temple Mount and turned them on.
Here’s how they work: Muons have about 10,000 times the energy of a typical x-ray. They can easily pass through rock and earth—and anything less dense, like plants and people—but the denser the material they pass through, the quicker they lose their energy.
When muons hit the detectors with different energy levels, an image can be created of the density of the matter through which they passed. Empty spaces are easily distinguished. And archaeologists can “see” underground.
