Gas prices are still high, diesel is at a record high, and the reserve has shrunk more than 30% in six months. While the reserve held around 415 million barrels of oil in February, it now holds less than 285 million barrels of oil — lower than it has been since the early 1980s.

These rapid drawdowns of oil threaten the salt caverns' structure, says Samantha Gross, director of the energy security and climate initiative at the Brookings Institution, a nonpartisan policy think tank. As the reserves get lower and lower, the physical facilities can get degraded, Gross says: " You can damage them by pumping too much out of them."