Drive on Interstate 10 along the Gulf Coast of the United States, through Louisiana and into Texas, and you’ll see signs of the oil industry everywhere. There are offshore rigs out in the water and refineries lining the shoreline, where tankers deposit crude oil extracted from the Gulf floor.
But what you might not realise from looking at the surface, is that this area is also home to a network of underground salt deposits, known as salt domes.
And dug down deep inside them are caverns, carved out in the late 1970s, that contain the Strategic Petroleum Reserve, America’s huge stockpile of oil.
Current stocks stand at 374 million barrels worth of oil, well short of its capacity of 714 million barrels.
With the Strait of Hormuz now closed for more than two months, global oil supplies are being squeezed, with warnings mounting about shortages affecting global industries, from aviation to agriculture. In March 2026, as part of a co-ordinated move by members of the International Energy Agency to release 400 million barrels of oil to prevent price spikes, the U.S. began releasing 172 million barrels from the Strategic Petroleum Reserve.
Globally, at the end of 2025, global strategic oil stockpiles were estimated at 2.5 billion barrels, with China holding the most.
In this episode of The Conversation Weekly podcast, we speak to Scott Montgomery, a former petroleum geologist who lectures in international studies at the University of Washington, about why these oil stockpiles were built up in the first place, and how they work.
Montgomery says salt is an ideal place to store oil because it’s impermeable, but also quite movable under the right amount of pressure. “These are not huge open gaping caverns … we haven’t hollowed out the salt dome. We actually have 60 separate smaller what are called bottles, about 200 feet (60 meters) in diameter and up to 2,000 feet (610 meters) in vertical length,” he explains.
But Montgomery says that there is only a certain number of times that oil can be taken out and put back into the bottles. “These really have a safety margin of about five cycles of drawing oil out of them and putting oil back,” he says, without dissolving the sides of the cavern too much. “The geological reality … is that we’re going too have to make new caverns.”
Listen to the interview with Scott Montgomery on The Conversation Weekly podcast, where he traces the history of strategic oil reserves, and explains what happens when they run out.
This episode of The Conversation Weekly was written and produced by Gemma Ware, Katie Flood and Mend Mariwany. Mixing by Eleanor Brezzi and theme music by Neeta Sarl.
Newsclips in this episode are from DW News and CBS News.
Listen to The Conversation Weekly via any of the apps listed above, download it directly via our RSS feed or find out how else to listen here. A transcript of this episode is available via the Apple Podcasts or Spotify apps.
