The height of complete groundwater drainage above subsided longwall panels (referred to as H) at underground mines is determined using a data base of hydraulic head measurements made with multiple devices down the depth profile at each of a number of sites worldwide. H is shown to be relatively independent of most parameters except the geometry of the mined void and the overburden thickness. An empirical equation linking H to these parameters is developed using hydraulic head data, and confirmed using an independent data base of ground movement. H is shown to be the same as the height of the zone of major ground movement above a panel. H for special cases (above longwall chain pillars, above pillar extraction panels, and underneath significant water bodies) is invariably smaller than H above center panel for ordinary cases. A new caving model, from a groundwater perspective, is proposed for continuously sheared longwall panels at ordinary locations. It removes complexity and reduces the uncertainty in estimating H. The derived equation for H applies to a variety of strata types.