In open cast mining, the groundwater levels must be continuously reduced to protect the mining area from flooding and the excavation walls from instability due to water pressure. A large number of dewatering wells with submersible pumps are kept in operation for this purpose. A continuous water flow prevents the pump shaft from blocking due to the effects of hardening clay/iron oxide deposits. To regulate the pump output, precise level measurement is required in each of the up to 750 m deep wells.