Tel Aviv University scientists are using muon detectors to build a 3D map of undiscovered underground Jerusalem tunnels, chambers, and fortifications. Muons are a fundamental particle, created when cosmic rays hit our atmosphere. High muon levels indicate a cavity above.

https://www.timesofisrael.com/unable-to-dig-researchers-look-to-cosmic-rays-to-unlock-jerusalems-ancient-underworld/

https://cris.tau.ac.il/en/publications/muon-detector-for-underground-tomography