The Tour Isabelle double arch, also named the Tour Percée arch, is a double natural arch, located in Chartreuse Mountains, in the French Alps. Its span is 32 metres (105 ft), which makes it the biggest natural arch in the Alps.
It remained unknown for probably everybody, until its discovery in May 2005, when Pascal Sombardier, who was trekking to write his book Chartreuse inédite : Itinéraires insolites, dealing with lost places of the range, discovered it fortuitously. Its pictures illustrated the front cover of his book, published in 2006 : this double arch became then the symbol of the hidden treasures of the Chartreuse Mountains.
Located in a very remote area, with a difficult and dangerous access, very few guide books, even the most recent ones, mention its existence.