A river with two sources
The source of river l'Orbe is situated outside of Switzerland, in French Jura. The river flows through small Lac de Rousses, crosses Swiss border at the North-West, feeds beautiful Lac de Joux and... disappears. No out flowing waters from Lac de Joux can be seen and the lake is not big enough to evaporate it all. The answer is - river continues underground. Remember, the place is Swiss Jura, although not Alps, but still quite mountains. So the river goes under the rock, specifically under one of the highest peaks in Swiss Jura, called "Dent de Vaulion".
On its way though the mountain, the river found natural cavities, irrigated them further, thus creating complex system of caves and underground galleries, today famously knows as "Grottes de Vallorbe". The caves and galleries stretch to many kilometers sometimes completely under water. By today they are still not completely explored.
What is commonly known as "Source de l'Orbe" is not a true river source, but reappearance of l'Orbe waters in the place where it comes from under the ground (see pictures).
This little rocky hollow is squeezed in the end of short narrow valley surrounded by almost vertical walls of Dent de Vaulion.
Two pictures at the left were taken from Vallorbe Tourism Office web site. They, I must say, are not particularly spectacular. Probably they have been taken during summer time, when the waters were low. After rains this hollow looks like a rocket nozzle, throwing tons of water each second at an amazing speed. The water roars and boils; you hardly can hear your own voice. Water spray fills up the valley to the top. My picture on the right, although not supreme, gives some idea.
I guess the thing works as giant "water tower": as the water level in Lac de Joux begins to rise, masses of water start to pour over a natural spillway somewhere inside the mountain, filling in an invisible water tower. Comparatively small "nozzle" at the bottom can not cope with the tide resulting in the water level growth inside a water tower and corresponding development of pressure at the nozzle. Now, if I have not yet forgotten physics completely, each ten meters of water column give one atmosphere of pressure. The landscape in the place can easily afford not ten, but hundreds meters of water column.
This place certainly deserves a visit, especially considering that it will obviously be combined with the visit to the Grottes (the entrance to the Grottes is 20 m away from Source de l'Orbe).
There are two ways to the there: quick - by motorway and recommended - through the countryside. Choose the best for you, but mind you, going by motorway you save ten minutes, but loose a perfect opportunity to see Swiss countryside, and Jura. Also, if you like you can stop at La Sarraz and see the castle and museum in it. As you leave Vallorbe by the road to Lac de Joux valley, do not miss the small road taking you slightly left and sloping gently downhill. Watch for the road sign "Source - Grottes". Immediately after you cross the river by the bridge you will see a parking at the right. Leave you car there. Going on foot along a shady path you will reach the end of a little rocky hollow where the Orbe rises.
The other nice attraction of the area is trout fishing in the neighboring artificial ponds. It is not fishing in the normal sense of this word; it is more a dipping hook into the water and dragging fish out. But for little fishermen it is a great fun! No limits and cost is only CHF 15 per kilo - cheaper than in the supermarket.
For more information about the region have a look at the site of Vallorbe Tourism Office.