On one of our little trips in the country (this time with Niall and Antoinette) we visited a little chapel at Plaincourault, south of Angles sur Anglin. On the way back we stopped near Merigny to look at a weird thing.
Part of the Anglin river disappears underground south of Merigny, and rejoins north of the village. I'm not sure where exactly it leaves the main body of the river (and I am not sure anybody else is, either).
It was always thought that the water entering the river was a spring, but coloured dye testing proves that at least part of the water comes from the river upstream. The underground river was discovered by a caving group investigating the grotte de la Roche Noire (marked in red on the above map) in 1967. The full course of the river hasn't been mapped, because the cave has collapsed in places.
The spot where the water re-enters the river is far more obvious, and very picturesque. The water flows from under a small cliff into a pond, formed to stop the water eroding the far bank of the river.