Years ago while driving around the region around Beziers, France, we stayed in a room in a house in Montady, just north of this curious place , right by the edge of that wheel. We learned that the locals call it the 'camembert' since it looks like sliced wheel of that cheese.

It is called the Étang de Montady (Pond of Montady) and is clearly artificial. It had been a shallow valley that had no drainage so it flooded often. In the 13th century, local monks drained it by digging a trench going from the deepest point in the pond going south and then turns into a tunnel and crosses the hill that blocked the natural drainage of the valley to finally end in the next valley were it spills into a river that later flows to the Mediterranean. Later, more radial drainage canals were dug to facilitate drainage, giving it its bike-wheel shape. The line of the main drainage canal, going south and slightly east, is clearly seen in the map referenced above thanks to the maintenance road alongside. It is an open road, you can drive on it, I know because I did.

The hill at the end of that road is quite interesting, labeled Le Malpas in the center of this map. To the west, the archaeological site shows some Roman ruins placed at the very top of the hill were you have a good view of the valleys on either side. However, the most interesting part is the Malpas Tunnel, which actually should be plural, because there are so many …

The drainage canal arrives from the north, slightly west. That tunnel is the lowest level. It makes a V shape with another line converging on that spot which is a railway line that exits the hill at the bottom of the image. Then, coming mostly from the west and flowing east and slightly north, is the Canal du Midi whose tunnel was carved in the soft stone of the area in 1679.

Riquet, the designer of the canal, exploring the area to trace the path for the canal, learned about the drainage tunnel that the monks had dug several centuries before, which allowed him to assert that a tunnel could be dug easily and it would hold. This saved him a very long detour. He didn't want to run risks so he dug his tunnel right across the same patch of hill as the monk's tunnel to ensure that he had the very same quality of stone.

Finally, the road goes right atop the three tunnels.

It might sound strange that the water tunnel is relatively high, well above the modern rail tunnel that goes mostly level with the surrounding terrain. Riquet knew that he still had a long way to go to reach the Mediterranean and he didn't want to lose altitude because the canal would have to cross several high obstacles on its way to the sea. The water that feeds the canal comes from an artificial reservoir near Castelnaudary and it flows all the way to Sete. If Riquet let the height of the canal drop and then had to climb back over another obstacle, he would need to find another source of water at a good altitude to feed that separate section, something quite unlikely in the relatively dry region near the Mediterranean coast. The canal dates from before the era of the steam engine, and there was no way to pump water to an artificial reservoir, you had to take the water from wherever it happened to be available and then keep it for as long as possible.

Closer to the coast the canal finally drops several meters down near Beziers in the 9 locks of Fonseranes though it doesn't yet go all the way down to sea level. Actually, the north fork in the canal has two of the 9 locks leading to a canal draining in the river Orb. The problem was that, navigation wise, the Orb is too shallow, it serves the local traffic but if he wanted to reach the Mediterranean via the Orb, he would have to dredge it. So, the main southern branch goes on a bridge right over the Orb so it can completely bypass it and keep going. After a curve it enters and artificial basin that is the main river port for Beziers which, unsurprisingly, is close to the main railway station, as it happens in several places along the canal as its route is just as good for canals as it is for trains.

A side note: In these aerial views it is clear to see the oval shape of the locks in between the gates. They contrast with the ones in British boat canals, or any canal for that matter. Actually, the initial locks were just as squarish as any other but they soon discovered the flat walls collapsed into the lock. The walls are built with stones and water seeps through the gaps in between them and soaks the ground around it, which becomes unstable, heavy and fluid. When the lock empties into the lower canal, the pressure of that water on the stones, now no longer compensated by the water pressure on the inside, simply collapsed them. The locks are longer than those on the British boat canals and the jump much higher so the walls are subject to much stronger forces. That is why, very early on, they had to rebuild the locks with curved walls, much like gravity dams, to hold the weight of the soaked ground around each lock.

Riquet wasn't so lucky with the Libron river further east. The canal was already at the same level as the river so there was no way he could build a canal bridge over it so they had to cross it at the same level. The Libron, normally dry, also had the unfortunate habit of flash-flooding regularly. The problem was that the flash floods carried lots of silt and material that would block the canal if allowed to drain into it and would have required clearing regularly, so, a century later, they built the Ouvrages du Libron (Libron [water]works)

It is a set of two structures that have several sluice gates each with a calm lock in between. When the Libron is quiet, it just crosses the canal and each goes its own way. When the Libron is on a rush carrying lots of silt, they close off the canal and let it run through the gates on both sides. When a barge needs to go through, they close the sluice gates of the river on one side and open the canal on that side so the barge can go into the middle, calm section in between the two structures. Then they close of the canal again, isolating the barge in the middle lock, and open the gates for the river behind the barge. Then they close the gates on the river on the other side and open the canal, so the barge can get through. Thus, when the Libron is rushing through, the canal and the river never flow into each other and the river doesn't silt the canal. At the same time, the barges that would have to cross the rushing Libron are protected from its onslaught as it is diverted through one structure or the other.

It is curious that centuries later, the French failed in using the same scheme to get across the isthmus of Panama. They tried to use the same type of level canal without locks they had used in Suez in a terrain that was unsuited for that. It is not that they didn't have any experience solving it.

In the Canal du Midi they used the reservoir of Saint Ferreol which they build in the very rainy region of the Black Mountain, and it reaches the Canal du Midi near Labastide-d'Anjou almost 20km south-west of it. Much as Gatun Lake does in Panama, that reservoir feeds the canal at the highest point where the water later flows, through several sets of locks, down to the Mediterranean in the south and to the Garonne river, near Toulouse, in the north.

Just as the Panama Canal links two oceans, the Canal du Midi and the river Garonne link the Atlantic with the Mediterranean which, besides providing very valuable local transport of goods, a link in between the two main coasts of France itself, without having to go all around the Iberian Peninsula and the dreaded Gibraltar Strait a route controlled by neighbors not always friendly.