silo, watertight and airtight structure for making and storing silage. Silos vary in form from a covered pit, such as was used by the early Romans, to the modern storage tower, dating from the 19th cent. A silo may be made of wood, brick, reinforced concrete, metal, or tile blocks, and is sealed with earth, airproof paper, or plastic. Most of the more modern upright cylindrical silos are glass-lined and are considered the most efficient. Older or less expensive styles include the box silo, made of planks lined with heavy paper; the fence silo, with pickets arranged in a circle and lined with paper; the concrete-lined trench silo, into which dump trucks unload the chopped fodder; and the underground pit, built like a well or cistern.