(FORTUNE Magazine) – Major oil companies exploring the Gulf of Mexico have found some of the biggest U.S. reservoirs since Prudhoe Bay. But don't look for petroleum prices to drop anytime soon: The oil lies beyond the well-developed continental shelf, which is some 1,000 feet under water, at depths of 3,000 feet or more. Shell Oil and BP, in a joint venture operated by Shell, recently announced a huge find 130 miles southeast of New Orleans at a depth of 3,100 feet. Officially, Shell says that the prospect -- named Mars -- may contain more than 220 million barrels of oil and natural gas equivalent. But oil companies are exceedingly conservative in early estimates of their discoveries, and industry sources estimate privately that Mars may hold one billion barrels or more, about 10% of the original Prudhoe Bay discovery. This find is just the prelude. Roughly 60 miles to the northeast, Exxon and Conoco have found between 100 million and 200 million barrels at 4,300 feet. And Conoco and Oryx Energy announced striking as much as 150 million barrels 2,900 feet down. Amoco, Arco, Chevron, and Texaco are also exploring the area. Deep waters will call for new technology that will hold down costs. One money saver is the tension-leg platform. Instead of standing on a superstructure of expensive steel, it floats, moored to the bottom by steel tubes. Such rigs may work in water as deep as 8,000 feet. Another way to get oil out of the deeper Gulf areas would involve a so- called subsea system (see illustration). Here, oil flows from wellheads on the sea bed to a manifold, which regulates pressure, and then by pipeline to a traditional platform in shallower waters. The wellheads will be maintained by remote-controlled robot submarines that can replace defective parts.