Réseau de l'Alpe, Underground cave network in Chartreuse Mountains, France

The Réseau de l'Alpe is the largest underground system in the French Alps and the third longest in France, lying under the Chartreuse massif south of Mont Granier, between the Col de l'Alpe and the Col de l'Alpette. Its galleries run beneath meadows and forest in the Hauts de Chartreuse national nature reserve, across Sainte-Marie-du-Mont, Chapareillan and Entremont-le-Vieux. In 2018 it had 37 entrances, from the Grotte du Biolet at 1,741 m to the Gouffre de la Combe de l'Arche at 1,546 m. The shafts cut Urgonian limestone, while the collector, the Rivière de Jade, follows Hauterivian marls.

Exploration of the system began in the 1940s with the Biolet Cave, which early explorers reached at considerable depths. Systematic expeditions followed, gradually revealing the extent of the network and discovering new passages over the following decades.

The collector of the network is called the Rivière de Jade, a name given by the cavers who reached it in 1964. The system lies beneath meadows and forests of the Hauts de Chartreuse national nature reserve, within the Chartreuse regional natural park. Its waters resurface at the Cernon spring in Chapareillan, at 1,160 m altitude.

The Réseau de l'Alpe is not open to the public: its 37 entrances, from the Grotte du Biolet at 1,741 m to the Gouffre de la Combe de l'Arche at 1,546 m, are reached only by caving clubs with permits from the Hauts de Chartreuse nature reserve. Access on foot is via the Col de l'Alpe and Col de l'Alpette paths above Sainte-Marie-du-Mont and Chapareillan. The Cernon spring at 1,160 m in Chapareillan is where the network's water resurges.

An underground river called the Jade flows through the connected cavities and creates an aquatic landscape within the massive limestone formations. This water system makes exploration dynamic and shows how water has shaped this rocky landscape over millions of years.