In a cave and tunnel under ancient Jerusalem’s City of David, two machines the size of dressers have spent months tracking subatomic particles as they fall from space and penetrate deep underground. These fleeting particles, known as muons, travel at almost the speed of light and exist for fractions of a second. Observing them during their short life can help map areas that aren’t visible to the bare eye—in this case, an underground area near the site known to Jews as Temple Mount and to Muslims as Haram al-Sharif, a place whose religious significance means archaeologists try to avoid any unnecessary digging.
While most of the physicists working at the site are academics, several work for Rafael Advanced Defense Systems Ltd., one of Israel’s state-owned defense contractors, which has an additional agenda: refining the technology to help map the network of tunnels Hamas has built under the Gaza Strip. Rafael began talking with physicists at Tel Aviv University about using the technology at least a decade ago, says Erez Etzion, an experimental particle physicist and professor at the university. “Obviously the security needs of Israel triggered us to move in that direction,” he says.