How to get there
Take road S from Vallon and turn left towards Labastide de Virac. Shortly before the Goule campsite on the left, there is a road to the right. Turn into here and park. To find the cave, walk back up the road (away from the campsite), crossing the river. After the bridge turn right alongside the river (heading downstream) and follow the path to arrive at the base of a cliff containing the entrance.
Equipment
Wetsuits or neofleeces recommended! This is not only for warmth, but to prevent you from sinking in the deeper pools weighed down with SRT gear. In 2003 an inflatable boat was taken, which was good for the lakes and canals but proved rather difficult to manhandle back out again, since it refused to deflate properly; furthermore, it would not have fitted through the low duck in the first canal even had the water levels permitted this. In 2005, with lower water levels and a fully neoprene-equipped party, we opted just to swim, which worked perfectly well. Warning
Only descend in the best dry, settled weather -- great flood risk.
Description and rigging
Backup to tree followed by single bolt to descend waterfall. Rebelay off thread on the left (just next to the little chute) (or spit close to this) and swim across the pool; the line can be reanchored on the other side using a thread on the left wall (looking inwards). If the pool isn't there, you'll probably need a handline to get down to the floor. At the drop (4m), a Y-hang on the right wall permits a very short descent to a single bolt rebelay; head into the pool on the right. There is another spit ahead towards the left of the pool, in the roof, followed by two spits on the flat rock just as the shelf opens into the chamber at the top of the next pitch (7m).
Single anchor on the right for traverse line, continued from previous two bolts, and then large thread belay in roof (cunning tactics required to lassoo slings around thread in 2003; fixed tat present in 2005) for main hang. Continue downstream; just before the next pitch (Toboggan) climb up the obvious ledge to the left. Thread belay on the left just as you climb onto the ledge. Further along the traverse, a bolt in the ceiling followed by a small thread provide backups for a Y-hang from spits in the roof at the end of the traverse (newish-looking, use the good two out of a group of three). A couple of metres down, a spit on the opposite wall provides a deviation. Approximately level with the "real" pitchhead (where the water falls over), a thread on the same wall as the Y-hang provides a second deviation (a doubled sling with a krab is about right).
At the bottom of the pitch, follow the stream but climb to the right as it starts to descend. A smallish hole to the right leads through boulders to gain a higher level. Following this in the downstream direction leads to three alternative possibilities for a hang into the streamway below. A 20' ladder through the second hole would be a good choice. (Alternatively, in low water levels follow the passage straight down, rigging off a spit on the right and a humungous thread on the left wall.)
Afterwards, a couple of short climbs down lead to Passage de Joly, a fairly narrow canal-like obstacle. Twin spits provide a belay for a traverse line, which can be anchored at the other end (cross by back-and-footing to start with) to a rusty fixed hanger. The line isn't actually much use for the crossing! 84m of rope reaches from the start of the Toboggan ledge to this point, rigging the small hole with SRT instead of ladders (not recommended!). Of course, these can be dispensed with if water levels are low, or if you don't mind getting a bit wet.
Continue downstream to the Grand Lac. There's another option to traverse around the right wall here (fixed ropes in place in '03 and '05), or you can swim, or even use a dinghy. Leave the dinghy inflated and proceed until you encounter a short chute into a deep pool (Grand Marmite). At around this point a spit in the wall allows a rope to be rigged across the pool, which is useful as the Cascade pitch follows immediately afterwards; this is rigged from two spits on the right at the pitch head. Carefully descending to the right keeps you dry (same trick possible on ascent with care). A 25m rope suffices for the traverse and the pitch.
Follow the water down to the left at the bottom of the pitch and proceed down the streamway to the canal. (An inflated 2.3m long dinghy fits all the way from Grand Lac to this point.) Jump in the boat and paddle to reach a duck just around the corner.
In 2003 this was found to be sumped. In 2005, it was passable with around 20cm airspace; the canal continues for a total length of around 100m, after which an interlude of walking leads to a second canal of similar length. After this point the water sinks into the floor, and the cave continues as a stomp through large tunnels floored with gravel and rocks; this apparently continues for several kilometres.
It should be noted that if the water level is too low, it can be difficult climbing back out at the upstream end of the canal on the return journey! The easiest place to climb back up is on the right-hand wall (facing upstream).