Speleologists studying the Alto del Tejuelo cave (Cantabria. North of Spain) reported recently about 28 years of research exploring new areas in the cave complex.

With an explored part comprising 206 Km of galleries in a 626 m deep cave; the report placed the cave system as the longest in Spain and third longest in Europe. Mammoth cave (USA), the longest in the world, has 685Km. The cave complex comprises 7 zones connected by the speleologists, spreads under four valleys and can be accessed by 20 different pits.

Webpage of the speleologist club in Spanish with some photos and maps on pdf: http://altodeltejuelo.com/

See also https://en.wikipedia.org/wiki/List_of_longest_caves