After 150 million years of thrashing and roaring through limestone rock, the Orbe River is content to wind leisurely through three kilometers of caverns and caves, now known as the Grottes de Vallorbe.
Discovered in 1961 by divers exploring a siphon, the sight was made public in 1974 and have welcomed researchers and spectators alike to see the largest caves in the Jura region. Additionally filled with waterfalls, stalagmites, dripping columns, fistulas, gushing whirlpools, and immense yawning ceilings, the Grottes de Vallorbe are a sight to behold, and a frigid one at that. Temperatures inside the caves stay at a steady 10 degrees Celsius year round. Humidity takes over at a full 100%, not surprising as the hum of rushing water is a constant reminder that you are just meters above the largest underground river in Central Europe.
Recent technology has improved the experience for today's visitors: user activated lighting highlights unusual features of the cave one by one in a "light show", while duckboard passages allow for a full view of cave surfaces.
Somewhat bizarre is at the end of the tour in which a mannequin dressed in a stereotypical fairy garb welcomes visitors to view the mineral collection, or Fairy Treasure Trove. In any case, the collection is impressive with over 250 rocks and gems from around the world in a spaceship like setting.
Exploration and development in the caves continue to this day.
Also, if (on the way back to the car park) you take the public footpath that branches off to your left and goes over the stream, climb up it for 15-20 minutes, you will come to a couple of other caves. They are nothing like as spectacular as the grottoes, and very, very much smaller.