A warning before we start this piece, it deals with sensitive topics, particularly sexual assault. So your discretion is advised.

The Giséle Pelicot case gripped the world when her husband went on trial. Well a new CNN investigation has exposed an underground subculture, with men teaching and encouraging one another to drug and rape their partners while they sleep. 

Claire Byrne was joined by CNN's Paris Bureau Chief who is also one of the CNN investigative reporters behind this, Saskya Vandoorne...