Factory-based industry accounted for an estimated 26.5% of GDP in 2017. More than 75% of Cameroon's industrial strength is located in Douala and Bonabéri. Cameroon possesses substantial mineral resources, but these are not extensively mined (see ''Mining in Cameroon''). Petroleum exploitation has fallen since 1986, but this is still a substantial sector such that dips in prices have a strong effect on the economy. Rapids and waterfalls obstruct the southern rivers, but these sites offer opportunities for hydroelectric development and supply most of Cameroon's energy. The Sanaga River powers the largest hydroelectric station, located at Edéa. The rest of Cameroon's energy comes from oil-powered thermal engines. Much of the country remains without reliable power supplies.