Crude oil is unrefined liquid petroleum; it contains predominantly carbon and hydrogen in the form of alkanes (saturated hydrocarbons), alkenes and alkynes (both unsaturated), and aromatic hydrocarbons. The other components present in oil are sulfur, nitrogen, oxygen, trace amounts of iron, silicon, and aluminum. Large amounts of hydrocarbon contaminants are spilled into the environment as a result of various human activities. Major accidental spills from oil exploration sites, oil tankers, pipelines (underwater and underground), spent marine lubricants, and storage tanks have become a common occurrence. Petroleum refineries also generate sludge and other oily effluents. It is estimated that more than 2.5 million tonnes of used lubricating oil is unaccounted for in the United States alone, and the estimated annual oil influx into the ocean is about 5 to 10 million tonnes.