Start from the parking lot behind Vertes Sensations. 50m after the bridge over the Leysse, take a path to the right, heading north (with your back to the lake). Follow the route of circuit no. 4 to the ruins of the Villas-Doria (steep final climb on a path). You then take the Col de l'Epine road for 2km before leaving it on your right (short descent). You then take forest tracks. You'll pass between rocks via the Pas de la Crevasse before reaching the Col de l'Epine, then forest tracks and a pretty single track to the Col du Crucifix (915m).
From here, you can take the voie Sarde (on your right) to descend directly, or continue towards the Col Saint-Michel (903m) from where you can admire the lake. The link between the two passes is punctuated by a number of short steep climbs. Just before the Col St Michel, take care as there's a steep descent ahead. Caution is essential! (Only expert trialists will be able to ride their bikes downhill).
At the pass, you can enjoy a long descent, first on a single track (narrow path), then on the Sardinian Way to the "Allemands" parking lot. Continue downhill to "La Grange du Trieu". Take the 2nd road on your right and continue on this short stretch of asphalt. Watch out for the gullies across the road, which may surprise you.
The last part of the descent is on an uneven path alongside a stream, which is always a little slippery. Even if it's not steep, beware of carelessness! Stay focused.
Return to the starting point on the greenway that runs along the east shore of the lake.