In 1839, Crozet surveyed the Blue Ridge Mountains and determined that the best way to allow the Blue Ridge Railroad, an extension of the Virginia Central Railroad, to cross the mountain would be through a series of four tunnels (from east to west: Greenwood Tunnel, Brookville Tunnel, Little Rock Tunnel, and Blue Ridge Tunnel) near Rockfish Gap at Afton Mountain. The long Blue Ridge Tunnel was opened in 1856, although rail service did not begin until April 1858. At that time, it was the longest tunnel in the United States and one of the longest in the world. Dug a decade before the invention of dynamite it was considered to be an engineering wonder of the world and was less than a half-foot (15.2 centimetres) off perfect alignment, as construction had proceeded from either end. Upon completion of the tunnels in 1858, the Blue Ridge Railroad ceased to exist, becoming a part of the Virginia Central Railroad.