A septic tank is a holding container for a septic system. It is an underground tank that is typically made of concrete, plastic, or fiberglass. It is designed to hold wastewater from the home and allow the solids in the water to be broken down by anaerobic bacteria. The solids are broken down and then fall to the bottom of the tank, with the solid-free wastewater leaving the tank and going to the septic field where it is filtered through the ground.