The Canal du Midi runs for 240km from the River Garonne at Toulouse via Carcassonne to the Mediterranean at Agde. It was the brainchild of Pierre-Paul Riquet, a minor noble and tax collector, who succeeded in convincing Louis XIV (and more importantly, his first minister, Colbert) of the merits of linking the Atlantic and the Mediterranean via the Garonne.
The work, begun in 1667, took fourteen years to complete, using tens of thousands of workers. The crux of the problem from the engineering point of view was how to feed the canal with water when its high point at Naurouze, west of Carcassonne, was 190m above sea level and 58m above the Garonne at Toulouse. Riquet responded by building a system of reservoirs in the Montagne Noire, channelling run-off from the heights down to Naurouze. He spent the whole of his fortune on the canal and, sadly, died just six months before its inauguration in 1681.
The canal was a success and sparked a wave of prosperity along its course, with traffic increasing steadily until 1857, when the Sète–Bordeaux railway was opened, reducing trade on the canal to all but nothing. Today, the canal remains a marvel of engineering and beauty, incorporating no fewer than 99 locks (écluses) and 130 bridges, almost all of which date back to the first era of construction. The canal has, since its construction, been known for its lovely place trees lining the riverbank. Sadly, a wilt infection was discovered in 2006 and since then the trees have systematically been cut down. They will be replaced however, the view will not be the same for many of the years to come. You can follow it by road, and many sections have foot or bicycle paths, but the best way to see it is, of course, by boat. Outfits in all the major ports rent houseboats and barges, and there are many cruise options to choose from as well.
Le Boat (leboat.co.uk) or Locaboat (locaboat.com), both have a number of branches in Languedoc and the Midi. For a green option, try the solar-powered 14-berth barges (accessible for passengers with mobility issues) available from Naviratous (naviratous2.com).
Voies Navigables de France, 2 Port St-Étienne in Toulouse (vnf.fr); they also have English-speaking offices at the major canal ports.