From the viaduct, walk along the river Ain and its fishermen's huts. Next to a small stone hut just a stone's throw from the water, you can discover this small stream which flows out of the rock to join the Ain.
The "Black Fountain" is part of the ENS Haute Vallée de l'Ain, which will be labelled "ENS" by the Department of the Ain in 2019. Real natural and landscape jewels of the Ain Department, the sites with the ENS "Sensitive Natural Areas" label are managed in such a way as to preserve the biodiversity they host and are developed for opening to the public (except in exceptional cases due to the fragility of the environments).
During a survey of the banks of the Ain in 1976 the cavity was discovered by speleologists. Robert Durand and Jean-Louis Fantoli explore 402m of drowned conduits up to a narrow laminar.
On 05/06/1977, an interclub operation (Daniel Andrès, O.Bigot, Jean-Louis Fantoli and Pierre Licheron) allows to find the continuation in the siphon and to open up in an exondated gallery finished by a hopper.
The cavity is mainly composed of two parts, characterised by the existence of a siphoning branch at the bottom of the underground path and a semi-active part at the top.
The entrance basin plunges to -3 to the start of the gallery.
After a low point at -7, the drowned corridor (2.5 x 5m section), with a surprisingly flat and smooth floor and ceiling, goes up after about fifty metres into a collapsed room, partly excavated at low water. Large boulders and slabs of rock block the passage and thick clay deposits line the floor and walls at this point.
Upstream, the conduit submerges completely and plunges to -10m. All along the way, stalagmites and stalactites attest to an exundated phase of the cave, and remain one of the main curiosities of the Fountain.
At 170m from the entrance, a 2m diameter shaft is cut out in the middle of the gallery, which is extended by a 30m long roller.
The shaft gives access at -20 to a low gallery also connected to the rolling mill by an 8m chimney. Erosive slabs line the shaft up to 280m, in a room at -18.
There are two options:
- at the back of the room, high up, a rolling mill at -10m gives off a perceptible current.
- To the west, a high diaclase crosses the axis of the room perpendicularly and opens up into the open air by closing in. At mid-height, at -5, a gallery extends for about fifty metres to the open air, 350m from the entrance.
The flooded gallery runs north for 284m. The overflow water from the underlying circulation flows through it. It develops on a stratum joint (4 x 1.5m), intersected by numerous perpendicular fractures. A 6m step precedes a lake with an impenetrable hopper at its end.
Water temperature: 10°C.