In the S&P article linked by another comment, there is an interesting note about storing other materials: The [Early Storage Reserve] caverns at the Bryan Mo

In the S&P article linked by another comment, there is an interesting note about storing other materials: The [Early Storage Reserve] caverns at the Bryan Mound site were originally used by Dow Chemical to store magnesium. https://www.spglobal.com/energy/en/news-research/blog/crude-oil/061516-a-rare-tour-of-the-strategic-petroleum-reserve https://www.spglobal.com/energy/en/news-research/blog/crude-...