The Ohio Statehouse construction began in 1839 on a 10 acre (40,000-m²) plot of land donated by four prominent Columbus landowners. This plot formed Capitol Square, which was not part of the original layout of the city. Built of Columbus limestone from the Marble Cliff Quarry Co., the Statehouse stands on foundations 18 feet (5.5 m) deep, laid by prison labor gangs rumored to have been composed largely of masons jailed for minor infractions. The Statehouse features a central recessed porch with a colonnade of a forthright and primitive Greek Doric mode. A broad and low central pediment supports the windowed astylar drum under an invisibly low saucer dome that lights the interior rotunda. Unlike many U.S. state capitol buildings, the Ohio State Capitol owes little to the architecture of the national Capitol. During the long course of the Statehouse’s 22 years of construction, seven architects were employed. Relations between the legislature and the architects were not always cordial: Nathan B. Kelly, who introduced heating and an ingenious system of natural forced ventilation, was dismissed because the commissioners found his designs too lavish for the original intentions of the committee. The Statehouse was opened to the legislature and the public in 1857 and finally completed in 1861. It is located at the intersection of Broad and High Streets in downtown Columbus.