tl;dr: The US Strategic Petroleum Reserve stores 714 million barrels of oil in 60 underground salt caverns along the Gulf Coast, created by injecting fresh water into salt domes to dissolve out cylindrical voids. Salt's low permeability and self-sealing properties contain the oil without steel liners, and withdrawal works by pumping water into the bottom to float oil out. At ~$3.50/barrel of capacity versus $15-18 for aboveground tanks, it's far cheaper and more attack-resistant than surface storage or excavated tank facilities like the Navy's now-defunct Red Hill site.