Overhead power lines are used to transmit energy over large distances. They are made of one or more conductors that are suspended by towers or utility poles. Overhead power lines are typically the lowest cost method of power transmission when it comes to large quantities of electricity. The towers that support the lines are made of wood, steel, concrete, aluminum, or sometimes even plastic. Overhead lines are used with the goal of maintaining adequate clearance between the ground and the energized conductors. Underground just means that the power lines run underground rather than overhead.