the only country in the world that uses oil shale as its primary energy source. In 2018, oil shale accounted for 72% of Estonia's total domestic energy production and supplied 73% of Estonia's total primary energy. About 7,300 people (over 1% of the total workforce in Estonia) were employed in the oil shale industry. The state revenue from oil shale production was about €122 million. Mining Estonia has adopted a national development plan that limits the annual mining of oil shale to 20 million tonnes. If mined at this rate, mineable reserves will last for 25–30 years. In 2019, 12.127 million tonnes of oil shale were mined. As of 2021, five oil shale mines are in operation; three are open-pit mines and two are underground mines. The mines are owned by four companies. Several mining companies have applied for permits for opening new mines. Historically, the ratio of underground mining to open-pit mining has been approximately even, but usable deposits close to the surface has become scarcer. The Estonia underground mine at Väike-Pungerja, operated by Enefit Power, a subsidiary of Eesti Energia, 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. The Narva open-pit mine is operated by Enefit Power, and the Põhja-Kiviõli open-pit mine is operated by privately owned Kiviõli Keemiatööstus. Both mines use highly selective extraction in three layers of seams. The Narva mine uses a technology that involves breaking up both the overburden and the targeted deposits by blasting and then stripping the rock with relatively large-bucket () excavators. The company has applied and received a permit to start oil-shale mining in Narva by using underground longwall mining technology. The third open-pit mine, operated by Kunda Nordic Tsement which belongs to German HeidelbergCement group, is located at Ubja. Enefit Power and VKG Oil plan to open jointly a new mine in Oandu. Electricity and heat generation In 2020, 2,225 GWh or 40.3% of Estonia's electricity was generated from oil shale and 748 GWh or 13.6% was generated from oil