The longest and oldest canal tunnel is Scotland, although only two remain, the Prospecthill Tunnel (also known as the Hallglen or Falkirk Tunnel) dates from 1822. Designed along with the rest of the Union Canal, by Hugh Baird (1770 - 1827), this tunnel brings the canal from Glen Village into Falkirk under Prospect Hill. It was built only because William Forbes of Callendar House (1743 - 1815) was adamant that the canal must not cross his property. The tunnel portals are of dressed masonry, set within deep cuttings, but the tunnel itself is mostly natural rock-faced with the exception of masonry rings at each end, and masonry arches and patches of brick where loose rock was a problem. The tunnel is 636m (2088 feet) in length, 5.5m (18 feet) wide and at least 3.6m (12 feet) in height above the water (which is around 2m / 7 feet deep) but grows to almost cathedral-like proportions in some of the rock-faced sections. There is a tow-path, 1.5m (5 feet) in width, along the eastern wall. Construction through hard rock was a challenge and undertaken from either end but also from faces exposed at the bottom of intermediate shafts.
Lights were installed as part of the Millennium Link project in 2002, and improved in 2016 to show off details such as the holders for candles, which represented the original lighting system, dynamite stores and the shafts, making a walk through the tunnel a remarkable experience. Lighting has also enabled theatre performances to take place in the tunnel.
There were two tunnels on the Glasgow, Paisley and Johnstone Canal (1811), but these were lost when it was converted to a railway in the later 19th C. The only other canal tunnel in Scotland is the short modern Roughcastle Tunnel associated with the Falkirk Wheel.
