A method for manufacturing a tank for storing flammable and combustible liquids underground includes fabricating a cylindrical tank core. Structurally, the core encloses a chamber and has an outer surface. Further, the core includes at least one opening for monitoring the integrity of the chamber positioned on the core's bottom centerline. Also, the tank includes a screen attached to the outer surface of the core along the bottom centerline to cover the opening. Moreover, a foil is affixed to the screen and to the outer surface of the tank core. Sprayed or applied on the foil is a seamless jacket formed from polyurea, thermoplastic, polyurethane or polyamine epoxy to encapsulate the tank core.