Hidden chamber discovery in the underground Hellenistic necropolis of Neapolis by muography

We report in this paper the muography of an archaeological site located in the highly populated "Sanità" district in the center of Naples, ten meters below the current street level. Several detectors capable of detecting muons - high energy charged particles produced by cosmic rays in the upper layers of atmosphere - were installed underground at the depth of 18 m, to measure the muon flux over several weeks. By measuring the differential flux with our detectors in a wide angular range, we have produced a radiographic image of the upper layers. Despite the architectural complexity of the site, we have clearly observed the known structures as well as a few unknown ones. One of the observed new structures is compatible with the existence of a hidden, currently inaccessible, burial chamber.