The U.S. stockpiles oil in huge underground salt caverns. Here’s why

Inside the massive underground salt caves where the U.S. stashes its oil

Salt, with its ability to seal liquid in, is uniquely suited to storing the nation’s Strategic Petroleum Reserve

Since the war with Iran began, the Trump administration has drawn down the U.S.’s emergency oil reserve to near its lowest level since the Reagan era. In a bid to keep exports flowing and reduce domestic gas prices while a fifth of the world’s oil supply remains trapped behind the Strait of Hormuz , the administration has pulled out 66 million barrels and counting from the Strategic Petroleum Reserve (SPR), a set of colossal underground salt caverns in Texas and Louisiana that house the nation’s buffer of crude.

This raises certain questions. Could the depletion prompt a market panic , raising prices even higher? When will the reserve be replenished? And wait a second—we store oil where ?

Beyond its political and economic dimensions, the story of the SPR is one of geology. It begins not with the energy crisis of the 1970s, which spurred the U.S. government to create a stockpile of oil, but 160 million years earlier, during the late Jurassic.

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Around then, a rift formed between what is now the Gulf Coast and Mexico’s Yucatán Peninsula. As the land masses slowly drifted apart, a basin opened between them. For a time, this basin was isolated from the world’s oceans, so rainwater runoff from upland had nowhere to go—the water simply evaporated, leaving behind whatever dissolved minerals it carried. Chief among them was halite, the mineral form of sodium chloride, better known as table salt. After untold cycles of evaporation, the basin became a “big hole filled with salt,” says Mark Rowan, a geology consultant who specializes in salt tectonics.

Today the vast Gulf Coast salt deposits lie thousands of feet underground. At that depth and pressure, salt doesn’t behave like the seasoning in your kitchen, nor does it behave like other minerals. Most rocks are porous and permeable, meaning they’re filled with interconnected spaces that enable liquids, including oil, to seep through. But salt has special properties that make it impermeable—it flows and deforms, more like plastic than rigid rock, allowing it to self-heal incipient cracks.