New York to Los Angeles

Pittsburgh's skyline through one of its 446 bridges.

Houses were built at the top of the hills to extend the reaches of the city.

Downtown Pittsburgh through Smithfield Street Bridge, the oldest bridge in the city.

Around 20 bridges cross the Allegheny and Monongahela Rivers from downtown.

The Allegheny and Monongahela Rivers flow west, joining to form the Ohio River. Pittsburgh grew from the point where they meet.

c.1900 - The funicular railway, the track running up the slope in background of this picture, was once the only way of accessing the residences at the top of the hills.

Here it is possible to see the extent of the rivers that surround Pittsburgh, with the ridges of the Appalachian Mountains to the east

1828 - The city grew from the 'Golden Triangle', the point where the three rivers converge.

Once the Duquesne Incline was the only route to the residences at the top of the hill, but is now mostly used as a tourist attraction.

Pittsburgh's downtown now occupies the point.

The Sixth Street Bridge leading into downtown Pittsburgh.

View of Pittsburgh from the top of the Duquesne Incline, the last working funicular railway in the city.

The lights of Pittsburgh with the hills in the background.

1796 - Plan for the town of Pittsburgh, showing the viscinity of coal mines - they are marked at the south and north shores of the rivers.

Pittsburgh's steel industry fuelled the expansion of the railway.

Chimneys from the former Homestead Steel Mill have been incorporated into the city's new Waterfront development.

Some of Pittsburgh's factories are still in operation today.

c.1900 - Pittsburgh produced the most steel in the world from the late 19th century up to the end of World War II.

The US Steel Tower, Pittsburgh's tallest building, and headquarters of US Steel, which was founded in Pittsburgh.

c.1900 - One of the many furnaces that emitted the damaging smog.

The Bellefield Boiler Plant used to burn coal and natural gas, but now only produces steam through its chimneys from pumping hot water to heat the nearby university buildings.

Pittsburgh's steel production meant that its railroads expanded quickly, providing links to the rest of America other than by river.

An now-uncommon smog cloud engulfs modern-day Pittsburgh.

Pittsburgh's chimneys no longer spew out toxic pollutants but are still part of the urban landscape.

Even in the late 1950s smog was still visible on Pittsburgh's skyline. In the centre of the picture is Forbes Field, the city's old baseball park, which was demolished in 1971.

c.1900 - Pittsburgh's 'point' is just visible through the smog.

Though pollution levels are a fraction of what they were at the start of the 20th century, Pittsburgh is still one of the most polluted cities in America.

The Arrott Building is Pittsburgh's oldest skyscraper. Built in 1902, it would have suffered from the smog.

Like most of the city's stone buildings, the University of Pittsburgh's Cathedral of Learning is stained with pollution.

1874 - Emissions from burning coal was commonplace in the 19th century, as is shown by the plumes of smoke emitted by factories, and coal-burning boats.

1857 - Clouds of black smoke billowed from the furnace chimneys right across Pittsburgh.