When a tanker collides with a rock or another ship, an oil leak occurs. The oil leaking onto the surface of water forms a thin layer and, if not treated, is brought with the waves to the shore.
An oil leak can be removed in three ways. Mechanically, that is by putting special floating hose around the leak and sucking it from the surface, which is done by very expensive specialised ships. Chemically, that is by putting a detergent directly on a leak, which allowsthe oil to be dissolved in water and sink at the sea bed. Biologically, by use of special oil-eating bacteria which change oil into harmless substances such as water and carbon dioxide.
Only then the remaining pollution can be removed biologically.
The chemical method is the cheapest and the fastest one but it leaves a layer of oil on the sea bed, killing sea life there. The mechanical and biological methods are expensive and may take many days before the leak disappears but leave little ecological damage.