Description
One hundred years later, Anton Roschmann, a historian from Innsbruck, passed by it on his nature trail in South Tyrol, who mentioned a '... vast cavern that extends immeasurably near Castel Sporo' in his academic dissertation 'Regnum animale vegetabile, et minerale medicum tyrolense' (1738). Perhaps at that time, the castellans had already transformed the spacious entrance cavern of the Bus dela Spia into a storage area, although it is unclear at what time the underlying drainage tunnel was dug to prevent flooding of the entrance cavern by floods from the interior.
With the castle having fallen into disrepair in the 18th century and the old Roman road that connected Val di Non with the Giudicarie, through Andalo and Banale, fallen into disuse for centuries, the cave was not heard of again for a long time until, at the beginning of the 20th century, Cesare Battisti (who was an important geographer in the region) decided to go and explore it in detail and gave the first detailed description in his Guide to Mezzolombardo, accompanied by some photographs of the imposing stalagtites that adorn the largest lake.
At present, the Bus de la Spia is one of the most popular destinations for Trentino caving excursionists because of the ease with which one can get there (easily reached by car in just a few minutes from Sporminore), the beauty of the small lakes in the upper gallery, and the suggestive phenomena linked to the movements of the lake at the bottom of the cave.
A real mystery in this cavity is in fact the imposing water siphon that closes the gallery 300 metres from the entrance (about 56 m lower than the level of the road, outside), perhaps unique in Italy, which is triggered more or less regularly at certain times of the day, in every season and not only in the rainiest periods. The siphon is fed by unknown tunnels that it has never been possible to explore despite several attempts by experienced cave divers. When the water level in the innermost part of the siphon exceeds a certain threshold, it is triggered and begins to discharge outwards (i.e. towards the known part of the cave). Witnessing the phenomenon is quite exciting: at first glance, what appears to be a small, motionless lake begins to "swell" and overflow; the water slowly rises and invades the cave on a slight upward slope; loud noises begin to be made by the masses of air compressed in the side recesses of the main gallery, and by that time it is good to have already withdrawn a few dozen metres, at least until the beginning of the large, steep final landslide chamber. At the base of this, the waters normally stop and, after a short interval of time, resume their descent and flow towards the inside of the siphon, just as quickly and with even louder roars, leaving the gallery completely free and dripping at every point of the vault.
In periods of particular drought, the phenomenon may not occur or may occur in a mitigated manner (the water invades a smaller section of the tunnel, with less noise and less spectacularity). In times of exceptional rainfall, on the other hand, the water may rise quite a lot along the steep landslide hall and, once it reaches the upper horizontal part, even flow outwards. In recent times, the last flood that can be remembered, with water coming out of the entrance, occurred during the flood of November 1966, but even in more recent years, the water has managed to completely overtake the lower gallery and invade the lake gallery, up to a few metres from the entrance.
Several cave divers have attempted to explore the large siphon in the past, but all of them were forced to stop after a few dozen metres due to the movement of the water and the consequent dangerous pressure changes during the dive. The maximum limit reached is almost a hundred metres beyond the dive point and the cave is said to have several small branches, all underwater and naturally unexplored.
The visit to the Bus de la Spia takes no more than 2-3 hours and is worth it not only for the spectacle of the siphon (assuming one is lucky enough to witness it) but also for the beauty of the upper part of the cave, with its lakes and some impressive stalactites reflected in them.
We change at the entrance, which is very wide (10x15 metres and 5-6 metres high) and in front of which even a couple of cars can comfortably stop. A little further on we can still see the remains of a semi-delicate gate installed in the 1970s for reasons of safeguarding the cavity; at present it should always be open, although it is advisable, before a visit, to contact the Sporminore municipality by telephone (it seems that a new gate is about to be installed).
The fully illuminated entrance cavern deludes us for a few moments, but after a few metres it closes almost funnel-like and forces us to lie down to pass a short pebbly tunnel and enter the cave proper. The only life forms that have kept us company so far, some large grasshoppers of the genus Troglophilus, stop at this point and we continue on our own after a last look at our carbide lamps. We leap over a small jump of a few metres and crawl between a curious organ-pipe stalactite and a small but deep pool, taking care not to fill our boots with water; we observe with interest a clear fracture that cuts diagonally through the stalactite, indicative of very recent (geologically speaking) movements of the rock. A few more metres and here we are in front of the first pond that completely occupies the bottom of the gallery. We pull out the dinghy, which we will then need immediately on the next larger lake, and begin the transfer amidst the din of voices and the flashes of souvenir photos.
The second lake is quite long (more than 40 metres in length in the wettest periods) and we can attempt to pass it on the left side, if not in a dinghy, where the water is shallower. On the opposite bank, the cave rises slightly after splitting into two branches. The one on the right, half-constructed by a large and characteristic stalagmite column and largely invaded by water, rejoins the main branch in a loop after a few dozen metres; we climb to the left along the larger tunnel and soon reach the top of the large 'camerone' that descends steeply towards the last part of the Bus dela Spia. The descent becomes slow and cautious due to the extreme slipperiness of the muddy floor and the small and sudden jumps between the landslide blocks that completely occupy it. After about seventy metres we are at the bottom and the tunnel becomes sub-horizontal again; just a few more steps and we are on the shore of the lake-siphon: the dark, motionless water barely allows a glimpse of the rocky walls that are lost in the depths towards unknown parts of the mountain. We sit and wait with a good sandwich in our hands; if we are lucky, a few sudden noises and a sudden rise in the lake will announce the start of the siphon and the rising of the water, prudently advising us to return to the base of the large hall to watch the phenomenon from a quieter part of the cave.
MAIN DATA ABOUT THE SPIA (Sporminore / VT-TN-21)
Entrance altitude: 610 m
Height difference: -94 m
Real development: 520 m
For further information: Gruppo Grotte "E. Roner", SAT Rovereto; Gruppo Spel.o Trentino SAT Bindesi Villazzano
Bibliography:
1 ) Esplorazione subacquea del siifone terminale del Bus de la Spia / Mauro Bombardelli. // In: Atti del XII Convegno regionale di speleologia del Trentino-Alto Adige : Grigno, 7-8 September 2002 / [editing acts: Marco Ischia]. - Trento : Società degli alpinisti tridentini, Biblioteca della montagna, 2004. p. 115-119.
2 ) I misteri del Bus de la Spia: alla scoperta di uno delle grotte naturali più conosciute e più affascinanti del Trentino / by Paolo Zambotto. // In: Postergiovani. - Trento - A.6, n°30 (Nov.-Dec. 1998); p. 44-48