This paper presents the approach for assessing the stability of underground quarries through the numerical modelling and the monitoring of the stress-strain state of the rock mass and of the support structures. On the basis of the practical experience and long term observation on different case histories, the key issues for a safe and profitable exploitation are given. The procedure is focused on the Candoglia cavern (named Cava Madre), a large mining void due to the exploitation of an high quality marble used for the Milan Cathedral (Duomo di Milano). The importance of the monitoring system for the back analysis of uncertain rock mass parameters and for the operation controls is discussed and emphasized, in order to provide practical design suggestion