The use of monitoring systems to detect wall movements in surface mining has emerged in the last ten years as a standard work and safety practice. The development and optimization of remote monitoring technologies like radar interferometry have greatly contributed to the anticipation of slope movements and to the reduction of incidents and fatalities related to slope failures. The same has not happened in the underground mining industry, where deformation monitoring of openings is in most cases still demanded to contact sensors like extensometers, micro-seismic gauges, strain gauges, fiber optic systems, which can provide only point information. The new HYDRA-U (HYper Definition Radar - Underground) radar system, designed by IDS GeoRadar, is a submillimeter accuracy radar interferometer operating at MMW (MilliMeter Wave) frequency; the system is able to produce an updated radar map every 30 seconds providing both range and angular resolution by performing FMCW Arc SAR acquisitions. In this research work a HYDRA-U unit has been extensively tested to measure the monitoring performances of the system and assess the actual applicability of this radar solution as a real-time ground fall early warning system. First tests have been performed in laboratory conditions using corner reflectors to assess the performance in terms of range resolution and angular resolution. Moreover, a motorized corner reflector with a micrometric movement control has been used to estimate the accuracy of the displacement measurement. The last part of the test consists in a long-term deployment in underground mining environment. The system has been deployed in an underground mine tunnel. During the test, the HYDRA-U system clearly picked up a moving area; the analysis of the interferometric data shows an evident developing trend with marked accelerations and periods of relative steadiness, which can be correlated with the intensity of the local mining activity.