Dams and reservoirs in Laos are the cornerstone of the Lao government's goal of becoming the "battery of Asia". Hydroelectric power is a significant resource in Laos, with an estimated technically exploitable capacity of 18,000 megawatts (MW). In fiscal year September 2013–October 2014, Lao hydro power plants generated almost 15.5 billion kWh. Of this amount, nearly 12.5 billion kWh was exported, earning the country over US$610 million. By November 2014, just over 3,240 MW of the country’s hydro power potential had been commissioned. Most existing and potential dams are on tributaries of Laos's main river, the Mekong, and work has now commenced on two dams on the mainstream of the Mekong itself at Xayaboury and Don Sahong. The country's earliest major dam was built on the Ngum River in Vientiane Province, for the Nam Ngum 1 Hydropower plant. Completed in 1971 with Japanese aid, it flooded 370 km² of forest and farmland to create a large reservoir. The Nam Ngum 1 power station supplied the majority of electricity used in Laos until the end of the 20th century, and also exported energy to Thailand. The Nam Ngum Reservoir has become a recreation and tourism destination in Laos. Guest houses have been built on some of the lake's many islands, formerly the peaks of sub-valleys in the area, while two islands were used as prisons during the 1970s and 1980s. The reservoir was logged by divers using underwater cutting gear several years after inundation and a fishing industry also developed around the lake. The original Nam Ngum Dam, at 70 m high and with a crest length of 468 m is now dwarfed by a second dam on the other side of the reservoir, part of one of the five hydropower projects planned for the Ngum River. There are sixteen hydro power projects in Laos that use dams to store or divert water for electricity generation. The highest dam built so far is the 185 m rockfill and concrete face dam at the Nam Ngum 2 project. The largest reservoir in Laos is the 450 km2 Nakai Reservoir, created in 2008 when the 39 m high Nakai Dam was closed for the first time. The dam and reservoir store water for the 1,070 MW Nam Theun 2 Project, the largest power plant in Laos in terms of generating capacity. As with other hydro power reservoirs in the country, the reservoir shrinks