The Bush Dome Reservoir is a giant underground formation. So yes, it's being stored underground.

I was replying to the last question: "Why would it be expensive?"

The question wasn’t why filling it would be expensive, but why maintaining it once stored would be considered expensive.

I answered that. It wasn't expensive. Building up the stockpile was expensive (but only on paper; financed via treasury debt), but once stored, it required very little maintenance because it was all held underground in porous rock. The only real expense was maintaining the wells.