The Bournillon cave (French: grotte de Bournillon) is located in the ''commune'' of Châtelus in the

Isère Isère ( , ; frp, Isera; oc, Isèra, ) is a landlocked department in the southeastern French region of Auvergne-Rhône-Alpes. Named after the river Isère, it had a population of 1,271,166 in 2019.Vercors Massif The Vercors Massif is a Mountain range, range in France consisting of rugged plateaus and mountains straddling the ''département in France, départements'' of Isère and Drôme in the French Prealps. It lies west of the Dauphiné Alps, from wh ...

. Its entrance is high, and wide, and is the largest in Europe. It is one of the main

karst springs A karst spring or karstic spring is a spring (outflow of groundwater) that is part of a karst hydrological system. Description Because of their often conical or inverted bowl shape, karst springs are also known in German-speaking lands as a ''T ...

of Vercors, with a maximum flow rate of per second, which feeds a hydroelectric plant.

Description

The main passage may be followed for over a kilometre to the Salle Terminale. Two short passages lead off from here, both terminating in sumps, where the passage disappears underwater. A passage in the west wall of the entrance porch joins the main passage via a ledge after some 600 metres. After four or five months of draught the sump in the right-hand passage leading off from the Salle Terminale drains, allowing access to further passages upstream. This section is dangerous in the event of storms. The Salle des Centaures, a chamber , is followed by a bedding passage leading to another gallery: Minos Centre. The following Galerie des Champs Elyséens, some wide, splits, one branch finishing at Siphon Alpha and the other at Siphon Beta. These siphons continue towards the south-by-southwest for at a depth of in Siphon Beta, and for at a depth of in Siphon Alpha).

Exploration history

In 1897 a team led by French caver Oscar Decombaz (1866 - 1914) explored the grotto as far as the Metro Needle where the galley becomes a

siphon A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in a ...

. In 1942 André Bourgin and Roger Pénelon found the upper gallery and the lateral gallery. Attempts to dive at the Metro Needle siphon did not achieve significant upstream penetration. An exceptional drought in 1985 allowed Maurice and Franck Chiron to discover of new galleries, terminating a point higher than the cave entrance, at the two siphons Alpha and Beta. David Bianzani and other cave divers probed the siphons, but further upstream exploration remains.

Hydrology

Lower in the Bournillon Gorge is the Sources de Arbois, which is the normal resurgence for the catchment area, which constitutes much of southern Vercors: the high plateau of Vercors and the Vassieux High Plateaux. When the Sources de Arbois flow in excess of per second, the overflow goes into the Bournillon system. When the flow rate in the cave exceeds per second then another overflow starts at the Grotte de la Luire, some due south in the Varnaison valley.. The stream from the Bournillon flows into the Bourne, which is a tributary of the Isère river, and from thence to the

Rhône The Rhône ( , ; wae, Rotten ; frp, Rôno ; oc, Ròse ) is a major river in France and Switzerland, rising in the Alps and flowing west and south through Lake Geneva and southeastern France before discharging into the Mediterranean Sea. At Ar ...

Geology

The cave developed at the base of the

Urgonian limestone The Urgonian Limestone is a geologic formation in France. It preserves fossils dating back to the Cretaceous period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainm ...

formation. Dating one of the

stalagmites A stalagmite (, ; from the Greek , from , "dropping, trickling") is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites are typically ...

indicated that the

speleothem A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on ...

had been formed more than 350,000 years ago. The formation of the cavity, above the Bourne Gorge, dates to 3–4 million years before the present, to the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Caves of France {{Caving-stub