Strategic Petroleum Reserve

Sophia was just reading an article about the Strategic Petroleum Reserve of the United States (and a proposal to double it) and while I knew how much oil was being stored I had no idea about how it was stored, very very cool (from Wikipedia):

The Strategic Petroleum Reserve (SPR) is an emergency fuel store of oil maintained by the United States Department of Energy.
The US SPR is the largest emergency supply in the world with the current capacity to hold up to 727 million barrels (1.156E+8 m3). The second largest emergency supply of oil is Japan's with a 2003 reported capacity of 579 million barrels (9.21E+7 m3).

The reserve is stored at four sites on the Gulf of Mexico, each located near a major center of petrochemical refining and processing. Each site contains a number of artificial caverns created in salt domes below the surface.
Individual caverns within a site can be up to 1000 m below the surface, average dimensions are 60 m wide and 600 m deep, and capacity ranges from 6 to 37 million barrels (1 to 4.3 million m³). Almost $4 billion was spent on the facilities. The decision to store in caverns was taken to reduce costs; the Dept. of Energy claims it is roughly 10 times cheaper to store oil below surface with the added advantages of no leaks and a constant natural churn of the oil due to a temperature gradient in the caverns. The caverns were created by drilling down and then dissolving the salt with water.