This is a post without pictures. I only can refer you to above video on Daily Motion, a 11.05 min video entitled La grotte à Maurice, made by Julien Maguin and Philip Lespinasse, intended as a documentary on an aspect of outsider art.
Maurice Dumoulin (1907-?) from Switzerland is the leading character of the movie. He was filmed when he was almost 100 years old, and revisited a grotto he had digged out himself. According to the Lausanne Art Brut Museum he has been active in digging the grotto from 1975 until 1995, according to a regional newspaper (meanwhile no longer available) he has done this from 1978 until 1984.
Anyhow, digging out the grotto was an enormous project. The grotto in the woods near Bussy, a small community in the canton of Fribourg in Switzerland, is 16.5 m long, 2 m wide and about 2 m high, which means that he must have removed singlehandedly and with simple tools some 60 cubic meters of rubble.
Dumoulin transformed this grotto into kind of an art environment by filling it up with an altar and a huge collection of all kind of objects he brought there, bottles, newspapers, ceramics, iron utensils, objets trouvés one could say, stuff that should have been discarded from another point of view.
I am not sure if this collection of objects had creative aspects. The internet hasn't evaluative descriptions and Dumoulin, when filmed, was probably too old to be coherent in explaining what this was all about.
As far as I understand the shots of Dumoulin revisiting his grotto have been done in the autumn of 2005; the film was edited in 2008.
The movie was part of an exposition in the Collection d'Art Brut in Lausanne entitled l'Art Brut Fribourgeois (febr-oct 2009), curated by Lucienne Peiry, at that time the director of the museum. So, with this imprimatur I suppose this grotto should be ranked as one of Switzerland's art environments.
On Google Books parts of the publication with regard to the exposition have been republished (with pictures by Mario del Curto of Dumoulin and the grotto)
first published may 2010, last revised sep 2016
La grotte á Maurice
Woods of Bussy, Switzerland