A ship canal tunnel is a dug channel through a mountain. Often, old canals connect one river to the watershed of another river, crossing over the water divide between the two rivers. To maintain a water supply at the highest part of the canal, sometimes the canal was built to a height where a natural watercourse could provide water. The remaining elevation difference of the water divide was then overcome by constructing a tunnel. Building the canal over the mountain would have faced the biggest challenge of water supply.
Tunnels can be relatively short, like the Malpas tunnel of the Canal du Midi, but they can also be over 5 kilometers long, such as the tunnel of Balesmes.