A large volume natural gas storage tank comprises a plurality of rigid tubular walls having opposing ends and intermediate segments with closed cross-sections extending along longitudinal axes. Each wall is interconnected at each end with respective ends of two other walls such that interconnected interiors define an interior fluid storage chamber. Exterior surfaces of planarly successive interconnected walls define sides of the storage tank. The tank further comprises exterior support structures each extending between the exterior surfaces of the walls forming each side of the storage tank and reinforcing the storage tank against dynamic loading from fluid in the interior fluid storage chamber. The tank further comprises closure plates each extending at least partially across exterior surfaces of the exterior support structures. Interior surfaces of the closure plates, interior surfaces of the exterior support structures, and exterior surfaces of walls at least partially define an auxiliary fluid storage chamber.