The owner, Veolia Environnement, one of the giants of the industry, has built an underground wall of similarly waterproof clay around the site which extends deep enough to reach the natural barrier below.
Goffin, who told the crowd after the match that he had pictures of Federer on his wall as a child, was ranked 109th in the world when the clay-court tournament started but he will be up to 68th when it ends next Sunday.