We drill to extract oil: oil under pressure often rises to the surface. Oil is lighter than rock overtop and wants to rise it. Forms underground and finds its way through cracks into atmosphere. Oil and natural gas often occur together in conjunction with seawater. Primary extraction is the initial drilling and pumping of oil. We force out oil out of a deposit majority all found already. Oil pumped out very simply by itself with little pressure gushes out. Secondary and tertiary extraction uses solvents, water, or stream to remove additional oil; expensive. Getting it out by injecting solvents into ground and then can extract it. Deposits much less rich are only left. Not easily pumped out typically something injected to pump out oil: ex. seawater injected to loosen particles and pump out oil. Drilling also takes place on the seafloor, mainly on continental shelves. It can be hazardous to drill in very deep water.