The Nantes-Brest Canal was commissioned in 1842, after more than 30 years of work carried out in appalling conditions by convicts and prisoners of war. At the time, it was a 360km long waterway with 238 locks (18 of which were swallowed up when the Lac de Guerlédan was created) and with a 555m difference in height. A total of eight rivers were canalised.