Labyrinth by Kate Mosse

Mosse's great page turner opens in the French Pyrenees in July 2005 when Alice Turner, a volunteer at an archaeological dig, spots a belt buckle and on digging it out dislodges a large rock which falls away to reveal the entrance to a mysterious cave.  Inside are the skeletons of two people lying on a labyrinth and Alice's story becomes increasingly intertwined with that of Alais, an 11th century noblewoman caught up in the persecution of the Christian sect of Cathars by the Catholic Church.  A really good entertaining read