French mountaineer Pascal Sombardier knew his high-altitude neck of the Alps well. Or so he thought. In 2005, when he was out on a hike in the Grand Manti section of the Chartreuse mountain range, researching locales for a book on unique destinations in the area, he was stunned when he spotted what looked like a giant double arch.

Sombardier took some photographs and shared them with the mountaineering community to learn more about this formation. Nobody had ever recorded it before, and through his discovery the world came to know of the existence of the natural wonder, dubbed the Tour Percée double arch.

Spanning 105 feet, the huge natural arch, also called Tour Percée, is the largest in the Alps, but it remained largely hidden because of its geography. There were no paths leading up to the base of the arch, and the structure itself lies within a bowl, making it invisible from below. While viewing it from the air, people often spotted the tower but the arch itself was out of sight.

Even though it’s been over a decade since it was first spotted, it is still largely off the grid and extremely difficult to access. Of course, Sombardier included the arch and the story behind its discovery in his book, with one of his photos of the arch splashed across the book cover.