The engineering behind the US Strategic Petroleum Reserve

One of the most fascinating things I've learned about recently is the engineering behind the US Strategic Petroleum Reserve. Here are the key requirements it's designed to meet:

The actual solution, like many great engineering solutions, is incredibly elegant and simple. But before we talk about it, let's start with how you might go about solving this:

Looking at the requirements, the first thing most people would probably think to do is to just take what we do commercially for storing petroleum and scale it up. External floating roof tanks (EFRTs) are the common solution for storing petroleum. Most tanks are confined to about 50 feet tall and 300 feet in diameter - larger than that and you start to have engineering problems with the foundation and drainage systems. This gives a volume of $V = \pi (150 \mathrm{ft})^2(50 \mathrm{ft})$, or $3{,}532{,}500 \mathrm{ft}^3$, equivalent to about 630,000 barrels of oil.