The third thesis holds that the river rises on the slopes of Pic Aneto at above sea level and flows by way of a sinkhole known as the "Forau de Aigualluts" () through the limestone of the Tuca Blanca de Pomèro and a resurgence in the Val dera Artiga above the Aran Valley in the Spanish Pyrenees. This underground route was suggested by the geologist Ramond de Carbonnières in 1787, but there was no confirmation until 1931, when caver Norbert Casteret poured fluorescein dye into the flow and noted its emergence a few hours later away at "Uelhs deth Joèu" ("Jove's eyes" ) in the Artiga de Lin on the other side of the mountain. From Aigualluts to the confluence with the main river at the bed of the upper Garonne valley at above sea level, the Joèu has run for (16 kilometres more to get to the French border), carrying of water, while the main river is carrying .