Buried underground in caverns dug out of salt on the Gulf coast of the US are millions of barrels worth of crude oil. This is the Strategic Petroleum Reserve, built up in the late 1970s.

Globally, at the end of 2025, global strategic oil stockpiles were estimated at 2.5 billion barrels, with China holding the most.

With the Strait of Hormuz now closed for more than two months, global oil supplies are being squeezed. In March, 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 US began releasing 172 million barrels from its strategic reserves.

In this episode, 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.

This episode was written and produced by Gemma Ware and Katie Flood with production assistance from Katie Flood. Mixing by Eleanor Brezzi and theme music by Neeta Sarl. Read the f ull credits for this episode and sign up here for a free daily newsletter from The Conversation.

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Voices of the South

The salt caverns used to stockpile oil

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The salt caverns used to stockpile oil