The Beaufort Sea contains major gas and petroleum reserves beneath the seabed, a continuation of proven reserves in the nearby Mackenzie River and North Slope. The Beaufort Sea was first explored for sub-shelf hydrocarbons in the 1950s and estimated to contain about of oil and of natural gas under its coastal shelf. Offshore drilling began in 1972; about 70 wells were set up by the 1980s and 200 wells by 2000. These activities resulted in dredging of about 46.5 million m3 of sea bottom soil, as well as discharge of drilling muds which contained barite, clay, caustic soda, and heavy metals zinc, copper, lead, chromium, cobalt, nickel, cadmium and mercury. About of oil was produced in 1986.