Tube-lev, or tube train, tube-lev car, or tube mag-lev are the terms used to refer to subterranean mag-lev train systems, commonly used on Earth and the Luna as mass transit. Tube-lev trains are extremely fast and constructed in tunnels which are kept at or near vacuum and combined mag-lev systems. The cars encounter virtually no friction or air resistance. Tube-lev or tube-leving, used as a verb "He tube-leved to the Root." The New Angeles tube-lev system is known as the Metro.