These reservoirs are linked by a 2¾ mile (4.4 km) long, 15 feet (4.6 m) diameter low pressure tunnel,[2] which collects water from three intermediate streams.

From Loch Treig, a main pressure tunnel 15 miles (24 km) long was driven around the Ben Nevis massif.

This has a horseshoe shaped cross-section, with an equivalent diameter of 15 feet 2 inches (4.6 m), and lined with concrete to reduce friction.

[citation needed] Steel penstocks then convey the water down the western flank of Ben Nevis to the power house below.

The power house and smelter are located to the northeast of Fort William, on the eastern bank of the River Lochy, into which the tailrace discharges.

[5] The main challenge was the pressure tunnel from Loch Treig under the Ben Nevis massif, which commenced in summer 1926.

The tunnel was dug by hand through the solid rock, with compressed air drills used to bore holes for the explosive charges.

On 3 January 1930, a charge of 1.5 tons of specially prepared gelignite was detonated to blast out the remaining section of rock.

These intermediate points were located close to watercourses, which were later captured by a series of dams and contributed around 16% of the total water for the scheme.

[7] The power station and smelter were completed by the end of 1929, initially equipped with five 6,800 kW generators, connected to Pelton turbines.

[11] Construction of the reservoir also necessitated diverting 1.5 miles (2.4 km) of the West Highland Line along the shores of Loch Treig.

[11] The level of Loch Laggan was not raised by the dam, as this would have required significant diversion of the adjacent A86 road and would have also inundated valuable property.

[11] The third and final phase of the scheme was undertaken between 1941 and 1943, and further enlarged the catchment of Loch Laggan by harnessing the headwaters of the River Spey.

[15] In 2008, Rio Tinto Alcan awarded a contract worth €30m for the upgrade of the hydroelectric turbines to Andritz Tech Hydro.

[16] This was to replace the original twelve turbines, which were horizontal-axis twin-jet Pelton wheel machines driving two DC generators with a total capacity of 72 MW.

[16][17] Five new horizontal-axis Francis turbines driving AC generators were installed by 2012,[18] each rated at 17.3 MW giving a 20% increase in power output.

GFG Alliance, which incorporates SIMEC and the Liberty House Group, as the successful bidder, bought the Lochaber Smelter for £330 million.