Abstract: A leakproof modular manhole system comprising a plurality of separate cooperating, plastic form units that snap fit together in a vertical stack. The deployed system provides a complete replacement for an existing manhole, or forms an entirely new corrosion resistant and leakproof manhole. Each of the seamless units is preferably rotationally molded from polyethylene plastic. Each double walled unit interiorly defines a material receptive annular cavity. The cavity may be filled with structural or non structural fill material. A generally tubular base unit is disposed at the bottom of the excavation for connection with the sewer line by an eccentric reducer coupling. A concrete invert is formed in the base in fluid flow communication with the sewer line. One or more tubular riser units extend serially upwardly from the base. The risers are available in varying lengths to accommodate different manhole depths. The system corbel is formed by a cone that structurally terminates alongside the pavement.