While women were barred from owning businesses in 19th Century France, three widows, who were exempt from the rule, created some of Champagne's most lauded empires. On the outskirts of the north-eastern French city of Reims, winding roads converge near a gated chateau. Cars line a roundabout enclosed by sprawling fields. The air is still, and it's calm. The real action is happening almost 20m underground. Carving through this underworld are more than 200km