Kongsberg Silver Mines
The silver mines of Kongsberg, in Buskerud county in Norway, constitute the largest mining field in Norway, with over 80 different mines. It was the largest pre-industrial working place in Norway, with over 4,000 workers at its peak in the 1770s and supplied over 10% of the gross national product of the Danish–Norwegian union during its 335-year-long history: over 450,000 man-years were expended in the production.
Oil shale in Estonia
The Estonia underground mine at Väike-Pungerja, operated by state owned Eesti Energia Kaevandused, is the largest oil shale mine in the world. The other underground mine, operated by privately owned Viru Keemia Grupp, is located at Ojamaa. Both mines use the room and pillar mining method. Oil shale mined at Ojamaa is transported to the processing plant by a unique conveyor belt. Although there are similar conveyors in operation in other countries, the one at Ojamaa is an unusually challenging installation since its path contains many curves and sharp turns.