Updated October 26th, 18th November 2001; 9th November 2003; 19th December 2008; 9th, 24th September, 10th November, 5th, 8th December 2015; 11th February, 29th April, 18th October 2016; 14th May 2019

   The Orillón system has three entrances and is normally entered via this, the middle one, as it requires no tackle. The usual approach to the entrance from the northwest is a hundred metre walk across a grassy field along the fence line. The entrance depression, in trees, is an impressive surprise. A walk down meets a small stream bed which enters the system through a hole on the east side. The cave is more easily entered a few metres away down a short climb to the right of the stream sink.
    The passage degenerates into a stooping rift which soon meets a streamway that runs from west to east. A stooping step on jammed blocks over a 5m drop meets a short easy traverse to the left and a climb down to the stream. Upstream leads to the Malbujero (1161) entrance and beyond, a solid choke with water emerging.
    Downstream, the passage is generally high with a couple of small cascades and over-wellie pools until a large passage is met on the left. This leads up to the base of the pitch from the third shaft entrance, Orillón (0023). (That entrance might be the most convenient if quantities of digging gear need bringing in). The remaining 250m of the cave is mainly walking. At the end, beyond a smaller section, in a high chamber with a large mudbank, the stream was said to "filter away through the floor, with a small length of high level passage above". However, during the 2015 resurvey, it appeared that the stream had excavated a route past the terminal mud bank. A short sideways squeeze enters a small chamber where it is possible to stand. A draughting route forward is a promising dig with possibly semi-open passage identified with a camera on a selfie stick. Nearly 5 hours of digging (team of 4 required) opened up the initial space in November. However, the final dig at Easter 2016 entered a chamber with a climb. Any leads at the top were too small. It became clear that all the recent digging effort had cleared away sediments that had been washed in since 1974, when the chamber had been entered. A sketch from the 1974 logbook shows the end reached in 2016.
   The stream sinks under the right hand wall in the large chamber and a short time was spent with a crowbar at Easter 2016 trying to open up a proper hole for the water to drain into. Work with a larger crowbar and digging down 1.5m through various sediment layers in the summer gave some incentive to pursue the dig - but some water diversion works will be required.

   A water trace at the end of November by Peter Smith from the end of the streamway tested positive at the Esquileña resurgence at Riva, and faintly positive at the eastern (north of Cueva Mortiro) resurgence in the Hoyo Mortiro (site 4272). Another test in January 2016 confirmed the results. The details are found here. The water resurges in the Hoyo Mortiro (site 4272) before sinking into the Cueva Mortiro (site 0005), but whether it passes through the Coverón streamway on its way is open to discussion and testing. The end of Orillón is more than a kilometre from the resurgence in the Hoyo Mortiro and more than 900m from the upstream end of Coverón. Altitudes would suit either scenario but the extended upstream end of Coverón apparently "finishes" at avens close to Torca XLs. (There are no inlets marked on the survey of the Sistema Mortiro-Esquileña.)
    Sima de Cagiga Redonda (0531) is a possible inlet to the system.

   J. Corrin found a stone axe about halfway along the Orillón streamway in 1996. The axe is 14cm long and 5.5cm wide, apparently of Galician origin, and was handed into the museum. According to Martínez Velasco Antxoka, 2001, the axe is made of ophite and is in such good, unused condition that it may have been grave goods. A line drawing from that publication is seen here.

   Ortiz (AM) lists three miriapods, collected in 1966 (Lithobius insignis Meinert, Lithobius derouetae Demange and Lithobius drescoi Demange), while Notenboom (AX) found the following fauna in 1984: Pseudoniphargus, Proasellus, Prosobranchia/ Hydrobioidea, Bivalvia/Sphaeriidae, Insecta and Oligochaeta. J. Corrin photographed and video'd a single Cantabroniscus in August 2015 (photo : video).
    Members of the Matienzo Karst Entomology Project visited the entrance passage and collected spiders and other bugs at Easter 2019.

References: Fernández Gutiérrez et al, 1966 (survey and photo); anon., 1974b (logbook); anon., 1974a; Cox G, 1973; Kendal Caving Club and Manchester University Speleological Society, 1975 (survey); Manchester University Speleological Society, 1982 (survey); Corrin J S and Smith P, 1981; Ortiz E, 1968; Notenboom J and Meijers I, 1985; anon., 1996a (Easter logbook); Ruiz Cobo Jesús and Smith Peter et al, 2001; Martínez Velasco Antxoka, 2001 (drawing); Ruiz Cobo Jesús et al, 2008 (drawing & survey); anon., 2015c (summer logbook); anon., 2016a (January, February logbook); anon., 2016b (Easter logbook); anon., 2016c (summer logbook); anon., 2019b (Easter logbook)
Entrance pictures : from 2004, 2015 and 2016
Underground picture(s): misc : final large chamber digging Easter 2016 : streamway Easter 2016 : dig in final chamber, summer 2016 : Easter 2019
Video: Activities in the cave, summer 2015 (YouTube) : Cantabroniscus (YouTube) : excavation November 2015 (YouTube) : Digging through to 1974 (YouTube)
streamway sink dig, summer 2016 (YouTube)
Detailed Survey : from 1965 low res  high res : resurvey 2015
Line Survey :
On area survey :
Survex file : yes : with CEZ, Coverón and Mortiro-Esquileña