A SEPTIC tank is a kind of tank used to store wastewater and sewage. It is typically buried underground and is made of concrete, fiberglass, or plastic. A scuba tank is a type of tank used by divers to store air underwater. It is usually made of aluminum or steel and is filled with compressed air. A THINK tank is an organization that researches and provides advice on public policy issues. It is typically composed of experts in various fields and is often funded by governments or private organizations.