Cardiff in making iron and later steel, brought to the sea by packhorse from Merthyr Tydfil. This was first achieved by the construction of a long canal from Merthyr (510 feet above sea-level) to the Taff Estuary at Cardiff. Eventually the Taff Vale Railway replaced the canal barges and massive marshalling yards sprang up as new docks were developed in Cardiff – all prompted by the soaring worldwide demand for coal from the South Wales valleys. At its peak, Cardiff's port area, known as Tiger Bay, became the busiest port in the world and – for some time – the world's
Cardiff Bay on the architectural map", housing exhibitions and visitor information. Cardiff Bay was used as the high-tech urban setting for the "Doctor Who" episode "Boom Town" and the show's spinoff, "Torchwood", whose makers deliberately avoided stereotypical portrayals of Wales in order to portray Cardiff as the modern urban centre it is today. In "Torchwood" series, there is a giant secret base underneath the bay, named "The Hub", from where the Torchwood team works. There is also a lift from the hub into the plaza with a perception filter making anyone who stands on the spot "not noticed". In the third series