Tunnel vision: Thirlmere Aqueduct brings fresh water to Greater Manchester
It's an everyday miracle: turn on a tap and out comes clean drinking water. But how does it get to millions of households in Greater Manchester? Step inside the Thirlmere Aqueduct.
Most people know that Manchester's drinking water originates in the Lake District.
But exactly how it makes the 84-mile (134 km) journey has been revealed by the water company United Utilities which has drained one of the UK's longest underground tunnels to allow engineers to carry out essential repairs.
The aqueduct is a 2-metre wide tunnel which runs underground all the way from Thirlmere Reservoir in Cumbria to Lostock water treatment works in Bolton.
While empty you could, in theory, start walking near Bolton and emerge just south of Keswick without ever seeing the sky above your head.
Thirlmere Aqueduct is an amazing feat of Victorian engineering.
THIRLMERE AQUEDUCT - FACTS
first opened in 1894 bringing fresh water from the Lake District
water moves at 4 mph, taking just over a day to reach Manchester
works by gravity alone, falling 20 inches per mile
when full, carries more than 200 million litres of water a day
was drained completely for the first time in 2005
It was built in 1894 to provide a continuous supply of fresh water to quench Manchester's growing population and its thriving industries.
Some sections of the aqueduct were painstakingly hewn through hillsides, creating spectacular cave-like tunnels.
The genius behind its construction, however, means that the water is moved by gravity alone without the need for pumping.
John Butcher from United Utilities said it had been a crucial part of the north west's water network for more than a century.
"That it is still in such good condition is testament to the ingenuity of our Victorian ancestors," he said.
"Since 2006, we have been carrying out an annual shutdown, to allow inspection and maintenance work to take place - ensuring the aqueduct is good for another hundred years.
"Even after all this time, it is still a thrill to walk the tunnels, and marvel at the skill of those early engineers."
As well as routine inspections, engineers will be strengthening the aqueduct floor directly beneath Bolton's football academy training pitches at Middlebrook, by pumping in thousands of litres of concrete.
The shutdown - part of an ongoing £23 million maintenance programme - will see it taken out of commission for the whole of October 2009, allowing engineers to walk parts of its length.
But Mancunians don't need to worry about the taps running dry - the city is tapping into supplies from North Wales for the duration of the shutdown.