In 1963 Fernández got in touch with Joaquin González Echegaray - a priest who was deputy curator at the Museum of Prehistory in Santander. In turn, Echegaray introduced him to Alfonso Pintó, the leader of SESS - la Sección de Espeleología del Seminario Sautuola. The Seminario Sautuola was an archaeological society based at the Museum and headed by its "director" or curator: Dr Miguel Angel García Guinea. The Sección de Espeleología was a newly-founded (May 1962) team of young enthusiasts who could carry out support work for the archaeologists, such as cave surveys and exploration. Indeed, Pintó was only about 17 years old, a student at the School of Commerce.
In September 1963 Pintó visited Fernández in Matienzo, catching the bus to Lastras and walking the rest of the way. Together they visited a few caves and began planning what turned out to be a series of expeditions through 1964 and 1965. Fernández was interested in carrying out a study as part of the thesis for his Geology degree; SESS were looking for a project involving the study of a group of caves which would be suitable for their club publication. A comprehensive report was eventually published, and this includes a diary-style "Historia de las Exploraciones", which gives a detailed account of their work. It naturally differs from the actual log book written up each day, often adding details, and sometimes omitting incidents.
The first expedition was from 12th to 19th July 1964, with nine members of SESS, two from the caving club belonging to the Francoist youth organization (O.J.E.), two cavers from Ramales de la Victoria, and assistance from locals such as Jesús Aja, and Antonio Santander Setién "Del Pontón", caving companions of Juan Carlos Fernández. The leadership was divided between Pintó, Caving, and Fernández, Geology. They had a grant of 10,000 pesetas from the Patronato de las Cuevas Prehistóricas (an organization linked politically to the Museum of Prehistory), which was quite a large sum, at least equivalent to a month's wages. The army lent them field telephones, and they had plenty of equipment - including as much as 220m of electron ladders - for both caving and scientific work. Although ladders were naturally the usual method of descent they also did some abseiling, using hemp ropes and karabiners.
The usual method of working was to divide into different teams, which often visited one cave in the morning and another in the afternoon. In this way they were able to cover an impressive list of sites. On the first day of caving, for instance, they visited Cueva del Agua, Cueva de Tiva, Sima-Cueva del Risco (i.e. Torca del Sedo), Sima de Andrés, Cueva del Volvo and Cueva de los Emboscados. In Agua they used inflatable dinghies, which punctured, and Antonio couldn't swim, so they had to abandon the cave by climbing along the walls. But that first day also gave them their first success, when in Risco Pintó located the main stream passage.
On following days they explored Sima del Burro (which has probably never been descended again since), Cueva de la Loca, Cuatribú, and Coquisera. In the latter they explored the main passages and dropped the 95m pitch in the upper gallery. Towards the end of the week they visited Rascavieja, Patatal, and Sima del Reguilón, and they returned to Risco, surveying a kilometre along the main passage.
The next expedition was from 27th September to 1st October, which gave the six members of this trip three days' caving. These were spent in Cueva de la Loca, Coquisera and Risco. In the upper gallery of Coquisera they used a step-ladder to climb a large flowstone which took them to the bottom of the daylight shaft. In Risco they continued surveying, and discovered the massive Sala Carballo.
Pintó and Fernández only had the assistance of two cavers and some locals for their third visit, from 2nd to 4th January 1965. They continued exploring Cueva de Tiva, and in Coquisera they traversed round the 95m pitch, using three pitons for protection. They came out of the cave at night, amid heavy snow.
On 18th April (Easter Sunday) Pintó and Fernández, joined by Alberto Alfonso, set out at 4 a.m. on the hopeless task of digging the choked end of Cueva del Concebo. After that they had time to visit other small caves, like Cueva de la Puerta, before attending Mass, and then go on to Sima de la Cabritilla. They surveyed this, obtaining 25.5m depth and 123.7m length. Unfortunately the survey published in Cuadernos II doesn't show much of these passages, although British exploration later confirmed the length.
In July 1965 SESS organised another large-scale expedition, but this time without the collaboration of other clubs. Their main objective was Sima-Cueva del Risco, which was to reach over 3km in length. Progress was hindered by bad weather - despite the season. They had heavy rain and storms, and kept in touch with Santander Meteorological Centre. Most of their work was done in the entrance maze, in the Sala Carballo area, the galería Pintó, and galería Arco. Given the conditions in which they were caving, it's easy to imagine the monotony, and after four days one of them wrote in the log-book, "the team is feeling desperate thinking that tomorrow they have to return to the same cave." After one trip they claimed to have been in "water above our knees" for seven hours.
However, they did visit other caves, briefly: they took photographs in Cofresnedo, Patatal, Agua and Tiva, the survey of Tiva was finished, and they tried scaling above Cuevona, the resurgence of Agua, finding that the visible entrances were choked.
They returned in September, to continue exploring, and to help make a short documentary. This was for NO-DO, the Francoist newsreel shown in cinemas before the main feature. Before filming began, they finished the Risco and Loca surveys. Then, on 24th September they filmed in Coquisera, the following day in Agua and Cofresnedo, and finally at Cuevona. After the filming, they explored Orillón and Selvijo.
By this time their study of Matienzo was almost complete, although Pintó was to return with various companions, "tying loose ends".
Over three days in October they explored Cueva de la Loca I, Jibero II, Gonzalo, Carrasquilla and Tizones. Pintó hurt his right leg on the first day, but recovered miraculously for the village dance on the 12th, a public holiday.
At the end of the month they were back to explore Jibero I, and Anderal I and II. Two friends of Juan Carlos Fernández' were also in Matienzo, and borrowed ladders and a rope to do some caving on their own. The following morning, 1st November, they hadn't returned. The cavers went in search of them, not really knowing where they had gone, and and finally found their rucksacks at the entrance of Cueva Coquisera. Entering the cave, they soon learnt what had happened: the two men had gone down the Chinas pitch and visited the lower passage. When the first man was reclimbing the pitch, unlifelined, he had fallen but fortunately his right foot had caught on a rung of the ladder; he was still there, 14 hours later, hanging upside down, unable to move, with the side wire wrapped around his ankle. By now he was in a terrible state, apart from the damage to his ankle and vomiting, blood had sunk to his head, which was badly swollen. Somehow the rescuers got a rope to him, which the man tied round his waist with a reef knot. As they pulled on the rope, the knot turned over into a slip knot, painfully tightening around his waist when he became trapped below an overhang. The man had to be lowered to the bottom of the pitch, where Pintó used the rope to improvise a harness around his waist and thighs, his chest and his legs, so that the man could be pulled up the pitch again in a more horizontal position. When he was finally at the top, Pintó returned to the village for help, and borrowed a horse to carry him down the hill.
As often happens, the cavers didn't want to give any publicity to the incident, and even in the log book the man is simply described as having hurt his ankle and unable to walk.
Because of the rescue, they didn't have time to explore, as they had planned, Simas del Ciervo.
At a committee meeting on 5th November, SESS accepted that the Matienzo study was finished, and made plans for expeditions to the Busta and Udias areas in the west of Cantabria.
Nevertheless, at the end of December, Fernández and Pintó were back to explore Cueva del Coverón, near Riba, and Selvijo and Anderal II in Matienzo. Once again they aimed to visit Simas del Ciervo, but ran out of time. In fact, this hole was never to be visited, and nothing more is known about it than the references to its name in the log book.
On 7th January 1966, they held the "premiere" of the documentary, called "Imagenes: Estudio de Espeleología" (Images: Speleological Study), in the Museum.
The first scenes of this black and white film are actually of the Museum, and the cavers are seen to be preparing their trip to Matienzo. The location then moves to the village centre, and they all walk up to the entrance of Cueva Coquisera (the only cave identified in the whole documentary). We see them abseiling, and collecting a cave shrimp. Then they descend the Chinas pitch by ladder, to the lower gallery, where they examine the bear skeleton. Without further explanation, the location now moves to Cofresnedo, and its massive formations. Here they carry out surveying and archaeological work before camping (in a tent) over night. In the morning they get out two dinghys and sail down the river (in Cueva del Agua). The final surface shots are of a diver failing to pass the sump in Cuevona, and the futile attempts of scaling above the sump pool. The newsreel therefore shows the variety of caving in Matienzo, but in a rather fictionalized way.
In July a group visited Matienzo to collect fauna. During three days they saw a dozen of the most easily accessible caves. The insects collected were sent to a geologist in Madrid called Ortíz, who later published his conclusions in Cuadernos de Espeleología III.
By this time SESS were struggling to get their own study ready for publication, which seems to have caused a lot of tension between the various people involved.
As it was, Cuadernos de Espeleología II came out with the date of 1966. Its 106 pages include the exploration diary, a description of the geology, hydrology and caves, and notes about archaeology and their food rations. In general Fernández gives accounts of the caves explored by SESS during this period, but survey details were sometimes omitted, as in the case of Sima de la Cabritilla. Other caves they'd visited had no surveys at all, but he does include caves he'd seen alone at some time, such as Cueva del Abono and Cueva-Cubio de la Reñada (the survey is dated 3/10/65).
Fernández also describes the dye-tests which were carried out: see the table below.
| DATE | DYE PLACED | DYE SEEN | TIME TAKEN |
| 12 April 1965 | Risco | Cueva Transformador, Cueva Tiva & Fuente la Lisa | 5hours |
| 12 April 1965 | Carcavuezo | Los Boyones | 10 hours |
| 28 September 1965 | Loca II | Cueva and regato de la Reguera | 2 days |
| 10 October 1965 | Selvijo | Loca I | 7 hours |
| 31 October 1965 | Orillón | Loca I (a light green colour) | 8 hours |
| 28 December 1965 | Stream in Hoyo del Mortiro | Resurgence between Riba and Ogarrio | 1 hour |
After this publication SESS virtually lost interest in Matienzo, although they were well aware that there was plenty left to be explored. Their surveys are full of question marks, some of which have been examined much later; such as the passage at the end of the galería Arco in Risco, in July 1994. No doubt other question marks still remain unpushed. But they had satisfied their original aim, to produce a study worth publishing. They had never intended to attempt an exhaustive study; as we know, that would take years. At the time many roads in the area had no asphalt, making Matienzo quite remote, especially for young people with no transport of their own. And the province was full of other caves to attract their attention.
Of course, it wasn't quite the end. Pintó returned to Coquisera several times; in December 1973 he traversed around the Chinas pitch and explored about 200 metres of passages on the far side.