Three centuries-old tunnels, including a medieval drainage system and a UNESCO-recognized canal, coexist beneath the hill of Ensérune, showcasing layered engineering history.
Three tunnels in one hill
On the hill of Ensérune in the Hérault department, three tunnels carved at different times are stacked within the same soft rock: a medieval tunnel that drains the Montady pond, the tunnel of the Canal du Midi, and a 19th-century railway tunnel. The oldest tunnel dates back to the 13th century.Tunnels at different depths
According to the Canal des 2 Mers cycling route, the Canal du Midi tunnel passes 10 meters below the surface, the railway tunnel passes about 20 meters below, and the Montady pond drainage gallery completes the stack at 30 meters. The tunnels cross each other rather than running in parallel, according to La Domitienne's information sheet.Medieval engineering
The medieval tunnel, the deepest, dates back to the 13th century and drains the Montady pond. The Canal du Midi tunnel was excavated between 1679 and 1680 and is 173 meters long, according to some sources, while an English source states 165 meters.Agricultural design
The Montady pond was dried up in the 13th century, according to La Domitienne's tourism office. The principle is based on radiating ditches, drawn from a central point to the ends of the area to be drained. Water converges toward this center and then drains through sixteen vertical wells spaced approximately 84 meters apart, until reaching an underground gallery that crosses the Malpas hill and passes beneath the Canal du Midi tunnel.Riquet's challenge
When Pierre-Paul Riquet arrives on the site, the hill already houses a tunnel that has been there for several centuries. This medieval structure may have even inspired the engineer, according to English-language documentation on Malpas. His construction site actually begins with an experimental bore above the Montady drainage tunnel before digging the canal bed.Engineering against the odds
Riquet excavates the Malpas despite objections. In 1679, Riquet decides to bore through the hill of Ensérune rather than going around it. Local tourist sources add that Colbert disapproved of this project, but that Riquet went ahead regardless. The rock presents a setback. After a few meters of hard rock, the workers encounter a very friable sandstone prone to slippage.A tunnel with a name
The tourism office speaks of a friable tuff, in which Riquet decides to bore despite diverging opinions. The name of the place, Malpas, comes from Occitan and means "bad passage." The tunnel is nevertheless excavated in record time. It is 173 meters long, offering an arch height of 8.50 meters, and 30 stone arches support it.A modern addition
In the 19th century, a third tunnel crosses the hill beneath the canal to accommodate the Béziers-Narbonne line. Excavated in the tuff, it crosses in depth the medieval ditch below and the Canal du Midi above, according to the Central Massif Railway Tunnels file.A layered history
The most striking aspect lies in the economy of locations: three construction projects separated by centuries chose the same hillside. The three structures still coexist, each at its level. On-site, the Maison du Malpas, near the site, offers guided tours and exhibitions.Preserving the past
A boat tour also allows visitors to discover the structure, and a towpath has been set up on one of the banks for pedestrians and cyclists. A hiking description mentions a protected route that follows the water inside the tunnel, a point to check before departing. The Ensérune oppidum, nearby, has more than 300 grain silos and water reserves carved into the very soft rock.A road through time
Above the three tunnels, the pass itself was crossed by the Via Domitia, the Roman road. The stack thus includes a fourth level of traffic, this time open-air, and it predates the three tunnels.
