A well with 5,000 feet of seawater and 15,000 feet of rock could have an overburden pressures at the bottom as high as 17,220 psi (5000 * 0.444 + 15000 * 1.0). That pressure is reduced at the surface by the weight of oil and gas the riser pipe, but this is only a small percentage of the total. It takes heavy mud (drilling fluid) inserted at the bottom to control the well when pressures are this high.