Storage tanks, for example, underground septic tanks, and their methods of fabrication are provided. The tanks include a substantially liquid impermeable plastic vessel having a top, a bottom, and sides； a concrete container encasing and retaining the plastic vessel, the concrete container contacting the bottom and at least partially contacting the sides of the plastic vessel； and at least one opening for receiving effluent, for example, waste effluent, into the plastic vessel. The methods of fabricating the tanks include positioning a plastic vessel into an enclosure； introducing concrete into the enclosure about the plastic vessel； allowing the fluid concrete to cure to provide a substantially solid concrete container about the plastic vessel； and removing the plastic vessel having a concrete container from the enclosure. Vessel shape retaining devices are also disclosed. Aspects of the invention are uniquely adapted to underground storage.
