Earthquake forecasting: a review of radon as seismic precursor

The first measurements of radon as a seismic precursor are dated back to 1927, but the first recording, that is reported in many publications, and that encouraged research on seismic precursors, was detected before the Tashkent earthquake of 1966. This paper is a review of the radon measurements performed all over the world, trying to distinguish between discrete and continuous measurements, and between measurements in soil, water or air. The role that the \"precursor radon\" had in the forecast of strong earthquakes in the past has been examined. The currently monitored sites in Italy are listed, and some of the results obtained are reported.