As navigation canals have to be built as horizontal as possible, as soon as topographical accidents appear the construction of tunnels is needed.
One of the best examples for this kind of tunnels are along the canal du Midi, in the South of France, built by Riquet during the 17th Century.
Another example was the Huddersfield Narrow Canal, whose works began in 1794. The direct route chosen involved a daring plan for a tunnel more than 3 miles long under the Pennines at Standedge - longer than any other canal tunnel at those times.