There are three tunnels under the Caledonian Canal, this is the most accessible because it is a short walk from Banavie Basin where there is parking.
Thomas Telford is always said to have had a heathy respect for water. One tunnel carries the Allt Mor under the canal and there is second tunnel for pedestrians and small vehicles (i.e. horse and car). The tunnel carrying the water is 106" wide and 117" high. The other tunnel is approximately the same dimensions but has a ledge each side, about 25" high and 17" wide. The two tunnels - there is about 74" between these.
