Vallorbes is a small town in the Jura mountains northwest of Lausanne, sitting right on the French border. It has long been a crossing point between the two countries, and the railway tunnel linking Switzerland to France through the Mont d'Or mountain emerges here on the TGV Lyria Paris-Lausanne line.
The town's most unusual attraction is the Fort de Pré-Giraud, a Cold War–era fortress built inside a mountain and disguised on the outside as a Swiss chalet. Inside, artillery positions and living quarters for over 100 soldiers are preserved as they were when the fort was operational. The Grottes de Vallorbes, a cave system carved by the River Orbe, is also worth visiting — the underground formations are impressive and the cave entrance is dramatically set in a cliff face, with the river flowing out of the mountain directly in front of you.
The Musée du Fer et du Chemin de Fer covers the town's ironworking and railway heritage, and is compact enough to visit alongside the caves in a half-day. Vallorbes is easy to reach from Lausanne by regional train and works well as a day trip.