The first report of this cavity was provided by members of GE Pedraforca, who explored and surveyed it in February 1970, with some later visits for fauna collection. Little more is known about it over the years, apart from some occasional attempts at locating it that had never been successful. It was not until October and November 2015 that members of the espeleodijous persisted in locating the pit and, after various attempts, succeeded. The pit is covered by large blocks, but they were able to open a way through even though a few metres from the mouth the passage becomes very narrow and they decided not to continue. The mouth was left protected by a large slab. When on 14 January 2016 the espeleodijous group returned to try to continue the exploration and collect the fauna trap they had left hanging, they were greatly surprised that someone had deliberately, with heavy machinery, filled in the pit with earth (see photo). One must go back to 1897 when N. Font i Sagué in his Catàlech, in the village of Montmany, cites a series of unpublished holes and says verbatim: 26) – Avenchs.- a) – Above Santa Eugenia, on lands of Castellar, there is a hole between the rocks, from which moisture comes out and a very deep noise is heard. With these few data it seems very likely that it is the avenc del Bac (it is the only one known above Santa Eugènia del Congost and beside the cingle del Castellar) and for this reason in the Catàleg Espeleològic de Catalunya (1980) it was renamed Forat or Avenc del Castellar. Despite this, we prefer to keep the name given by its first explorers and leave the other as a synonym.

Orthoptera

• Dolichopoda linderi Dufour

Coleoptera

• Speonomus urgellesi Español

Pseudoscorpions

• Neobisium (N.) ventalloi Beier

Isopoda

• Spelaenethes occidentalis n.sp. Vandel