I'm designing alternate means of storing energy off grid, and I'm having difficulty finding out how to figure out how deep a canister submerged under water would have to be to have a potentially energy of 3,000,000 foot pounds. For this, I would like to know the neccisary depth of a 500lb propane tank containing 400 gallons of air at 1 atmosphere would have to be to store 3,000,000 foot pounds. The tank weighs 1,200lbs. Any assistantce would be appreciated.