How to find it
Fontaine de Champclos
Take D901 westwards to Les Vans (very picturesque town and scenery). Right at first roundabout in town to reach another roundabout; left to arrive in town centre. Follow road through centre; just after leaving the town there is a turning on the left to Naves. Follow this until it bends around to the left, crosses a fairly long bridge and then heads uphill. Shortly afterwards, take a turning on the left which heads downhill; follow this until the tarmac ends and you arrive at a car parking area.
Continuing in the same direction on foot leads quickly to a sign telling one not to collect fossils; on the post next to this is a footpath sign. Take the path to the left adjacent to this, which descends to a gulley. Turn left, passing a building on your right, and then turn right up the main river. Shortly afterwards a path appears on the left (true right) bank to reach a tributary going to the left. Cross straight over (there is a yellow cross marked on a rock in the ground here) and pass along the right-hand side of a grassy area. The path continues, often accompanied by a black tube, until you have to cross the river just upstream of a waterfall. Continue further; as another waterfall comes into view down in the valley to your left, you will literally cross the obvious outlet from the cave on your right.
Grotte de Mont Blanc
From Fontaine de Champclos, head back towards the car park, and very shortly after leaving the entrance take the obvious path up the hill to the left. Follow this for a while, heading pretty much straight up the hill, to arrive directly at the entrance, a manhole at the base of the second small rock outcrop.
Tackle required for entry via Mont Blanc entrance: 20' ladder and line. Spits are in good condition.
Cowstails are probably advisable for the traverses, although most are easy enough just to use the fixed ropes as handlines.
This description goes from the Mont Blanc entrance through to the Champclos entrance, a good fairly short sporting through trip.
Descending the short entrance shaft leads to a spacious chamber; continue forward to the Gallerie du Lac. Traverse down, around to the left (lake visible in the dark hole down to the right), and then up again (fixed rope in place for whole traverse which was fine) to reach an impressive vantage point into the Mont Blanc chamber. The Mont Blanc itself is visible as you look out. Going down into the chamber, a large rift passage opens to the right. Following this leads, via a few small traverses (in-situ lines but they're easy anyway) and climbs, to a chamber where a narrow canyon leads off in the back left corner. Start up this but climb up almost immediately (obvious polished rock) and go through a small upward hole to emerge in a further chamber, with high ceiling. A passage around to the left at approximately the same level was not explored. To the right, a passage leads off which bifurcates and becomes narrower. Back at the exit of the hole, descending the slope (care -- loose rocks) leads to the bottom of the chamber, where it is possible to descend further to a small undercut containing some good formations. From here it appears to be possible to descend further into the boulders, but air quality was unclear and it looked unstable.
Returning along the rift passage to the Mont Blanc chamber, follow the right wall around to a sloping descent into a wide passage. This soon opens into a boulder-strewn passage which rises up and eventually chokes. Before the final slope up, a hole down to the left drops into a small streamway. Follow the stream (a bit wet!) through various small rifts and chambers to emerge at a more significant chamber, with well-proportioned stream passage leading off. This gradually becomes smaller, passing fine formations (care!), until an exit is made at Fontaine de Champclos. Exit via Fontaine probably not a good idea in wet weather, though it is unclear quite how much water ever comes out. Mont Blanc entrance looks bombproof.