Work has begun on the installation of seven miles of new pipes to help deliver drinking water to more than 165,000 people in Edinburgh and the Lothians.
The first cut was made at the foot of the Pentland Hills near Balerno.
The first few hundred metres of pipe has been installed.
About 75m (246ft) of pipe is being laid every day. The first sections measure 50cm (20in) in width and each weigh a ton.
The blue plastic pipe has covers on both ends to protect the inside of the pipe which are only taken off when the pipes are connected by hand.
A two-way treated water link will then be created between Marchbank water treatment works, near Balerno, and Glencorse water treatment works.
Construction is mostly taking placing on private land and is being carried out by Scottish Water's partner Caledonia Water Alliance.
Officials said the construction team had faced a number of challenges including overhead power cables and the military training area with its practice trenches which date back to World War One.
A tunnel crossing will also be dug under the A702 Biggar Road near Hillend to avoid disruption at the road when work takes place there next year.
The £20m investment will improve the water supply network by installing new mains, which will also have the capacity to link to other existing and future water supplies across parts of the south of Scotland.
Scott Fraser, Scottish Water regional corporate affairs manager, said: "It is exciting to see the start of this essential project to make the water supply for many of our customers more resilient.
"As always, we thank everyone impacted by our works for their patience and understanding."