Multi-fold increase (by an order) of amount of extensive seismic events (M ≥ 1) during sanctioned flooding of “Umbozero”mine, with mining excavations situated in the depth of 200-400 m from surface level are established by researches and shown in the paper. Relation between increase of seismic activity of a massif and time of mine flooding as weel as evaluation of danger of seismic events for neighbouring mine, surface objects and constructions are observed.