Industry has played a major part in Cardiff's development for many centuries. The main catalyst for its transformation from a small town into a big city was the demand for coal required 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 most important coal port. In the years leading up to the First World War, more than 10 million tonnes of coal was exported annually from Cardiff Docks. In 1907, Cardiff's Coal Exchange was the first host to a business deal for a million pounds Sterling. After a period of decline, Cardiff's port has started to grow again – over 3 million tonnes of cargo passed through the docks in 2007.