Yes, canal builders found it occasionally necessary to build tunnels. Some had towpaths and some didn't. In the old days, boats
were "legged" through tunnels without towpaths. One method was to lie on the roof of the boat and push
against the tunnel ceiling.
The longest U.K. tunnel of all:
Standedge Tunnel on the Huddersfield Narrow Canal- 3.24 miles (5.21 km) long. Reopened in 2001
The second longest U.K. tunnel that's still open:
Dudley Tunnel on the Dudley No. 1 Canal- 1.80 miles (2.90 km) long.
Mauvages Tunnel on the Canal de La Marne au Rhin in France- 3.09 miles (4.97 km) long.
Photo, right, courtesy of Xavier Girard.
As far as derelict tunnels in continental Europe are concerned, Antoni Vassileff tells me:
"The longest tunnel ever built in France was the 'tunnel du Rove', which used
to link Marseille to the Rhône river before a part of the vault collapsed in
the sixties (1963). The tunnel is 7 km long (4.3 miles), 22 meters (73 feet)
wide, 11 meters high (36 feet), 4 meters deep (13 feet). The opening of the
tunnel occured in around 1925 after twenty years of work (and a World War)."