In the Sedano y las Loras region, a short distance from Covanera (Spain), and quite well indicated is "El Pozo Azul", the longest underwater spring cavity in the world since 2009. It is an international reference in water activities and speleobuceo.
The region is located NE of the Province of Burgos (Spain) that, crossed by the Ebro River, is the southernmost sector of the Cantabrian Mountains. An enchanted labyrinth of rock, water and greenery characterize this zone.
In the 60's the first immersions began with the most rudimentary means, and it was not since the 90's when professional groups of English, Dutch and Spanish set their goal in this cave that today still does not know its end. Last summer a new record of 13,600m was reached, passing through numerous siphons (submerged areas in the water), and some meters with galleries where they place the base camps and use them as resting and material exchange points.
To get to this point of exploration, you have to pass long siphons (flooded galleries), among which there is, for example, a 5-kilometer-long and a descent of up to 70 meters deep; and another one of 3 kilometers long in which it goes down to 40 meters deep. After all, the well changes its appearance and opens to aerial galleries where you do not dive but walk, very narrow and ascending with very sharp rocks, which you have to put, locate and take sometimes very heavy equipment.
Toponomy of the explored cave:
In the following link you can see he expedition of the divers Rene Houben, Pedro Balordi y Jason Mallison: