Gold mines in South Africa have mining depths of up to ~4 km. Mining in such depths under high-stress conditions induces many earthquakes, which pose serious hazard to mine workers. To mitigate seismic risks and to understand the role ofmicroearthquakes in earthquake generation processes, we have conducted ultra-sensitive seismic observations, which allow acoustic emissions (AEs) to be detected down to M_W −5 (typical rupture size ~ several centimeters), at two South African gold mines. We have found out various modes of AE activity previously unknown from conventional seismic networks (detectability only down to M_W ~ −1): AE activity before and after an M_W 2.2 earthquake, AE activity likely reflecting damage accumulation processes ahead of an advancing mining front, and AEs occurring steadily along geological faults. This paper summarizes these findings, which received the fiscal 2016 frontier award of the Japanese Society for Rock Mechanics (JSRM).