outside the Houses of Parliament. The 20 ft (6-metre) high column was topped by a large gas lamp, and opened in December 1868. It was designed by the railway engineer J.P. Knight after a railway semaphore signal, with multi-coloured arms coming down to regulate traffic. Engineering 19th century London was the site of unprecedented engineering marvels. One of these was the Thames Tunnel, declared the "Eighth Wonder of the World" when it opened in 1843. Designed by Marc Isambard Brunel, it was the first tunnel in the world to be successfully built under a navigable river and took 18 grueling years to complete. Laborers employed to dig the tunnel, with the protection of Brunel's tunneling shield, endured five serious floods and multiple gas and sewage leaks, which caused numerous casualties and long delays. Although it was intended as an underground artery for the movement of goods between Rotherhithe and Wapping, it opened as a pedestrian tunnel during its early decades (there were 1 million visitors to the tunnel in its first 3 months of opening). Only in 1865 was it purchased by the East London Railway and converted for railway use. The Thames Embankment was one of the most ambitious public works projects in 19th century London. It transformed the appearance of the riverside between Chelsea and Blackfriars. There were three different sections: the Victoria Embankment, built between 1864 and 1870; the Albert Embankment (1866–70); and the Chelsea Embankment (1871–74). The embankments protected low-lying areas along the Thames from flooding, provided a more attractive prospect of the river compared to the mudflats and boatyards which abounded previously, and created prime reclaimed land for development. The Victoria Embankment was the most ambitious: it concealed a massive interceptor sewage tunnel, which channelled waste from a network of smaller tunnels away from the River Thames and out of Central London, towards the Northern Outfall Sewer at Beckton in East London. The Victoria Embankment also allowed an extension of the Metropolitan District Line underground to be built, from Westminster east to Blackfriars. In total, the Victoria Embankment reclaimed over 37 acres (15 hectares) of land from the Thames, allowing a broad east–west boulevard to be built and a series of public gardens. All three sections of the Thames Embankment are faced in grey granite, mounted with handsome cast iron lamps, and lined with London plane trees. In 1878, the lamps of the Victoria Embankment