The U.S. Strategic Petroleum Reserve is the largest hoard of emergency oil in the world, and the largest repository in the reserve system is stashed inside the Bryan Mound salt dome. Spread across 20 underground caverns, Bryan Mound contains about 250 million barrels of sweet and sour crude, the equivalent of 500 Panamax oil tankers.

The Strategic Petroleum Reserve was dreamed up in the aftermath of the 1973 to 1974 oil embargo that temporarily quadrupled gas prices in the U.S. Once prices stabilized, the Department of Energy bought up a vast sum of foreign oil, much of it at the now-bargain price of $28 dollars a barrel, and began looking for a place to pump it back underneath American soil.

Excavation of underground storage caverns at Bryan Mound presented a daunting challenge, but engineers came up with a clever solution using the nearby Brazos River. First, deep shafts were drilled down into the solid salt formation, and then water was injected at high pressure to dissolve the salt and open up voids. According to Bryan Mound Senior Representative Jorge Aguinaga, the resulting caverns were “2,000 feet in height and 200 feet in diameter, shaped like a cylinder.”

The water pumps also are the key to removing oil from the salt dome storage caverns. Water injected into the caverns at 1,000 psi sinks to the bottom, (remember, oil floats on water), forcing oil to come out the top at an equal rate. “Think of the caverns as a earthen vessel,” says Aguinaga. “When you put water at the bottom the oil has to go somewhere, and it travels out of the cavern through tubing.”

However, the act of flushing the caverns with water to remove the oil would have additional consequences on the facility. Every injection of water further dissolves the salt dome, and according to Aguinaga, there is only enough space for five drains and refills before the caverns would become structurally unstable. Of course, it is hard to imagine a scenario in which in which the SPR would be drained five times, so the facility is probably set for the foreseeable future.