Here you can find an interesting article about SC Avalon ‘s expedition to the Anialarra area at Pyrenees, which aim was to search for potential new entrances. The reason why they choose this season was that they wanted to sort out, which entrances are promising (draft blowing out). Because on average only 1 out of 100 pit reaches the depth of underground rivers (-400m) , it is more effective to focus on the blowing ones in summer. original here