The film documents the exploration of a grotto: the Bossi spring. The main characters are Beatrice Dell'Oro and Luigi Casati, two of the most courageous Italian speleosubs. One immersion after another the exploration pushes farther and farther from the entrance, but an unexpected event awaits the speleosubs. The grotto, full of water for the first 600 m, continues in air inside Mt Generoso. Problems connected with certain paramaters of immersion necessary to reach the aerial zones of the "Bossi" makes it a world first.