La cascade d'Aigualluts in the province of Aragon, Spain, nestles beneath Aneto, the highest peak in the Pyrenees at 3,404m. Standing at the foot of the waterfall at 2,074m altitude, we watched the autumnal sun rise, lighting up the rich colours of the deciduous trees dotted amongst the deep green of the stunted pines. We had arrived at the main source of the Garonne, France's 4th largest river.

The spectacular torrent is fed by glacial waters from Aneto, Barranca and Maladeta. A few metres downstream, the river disappears underground at the sink hole known as the Forau d'Aigualluts (or Le Trou du Toro in French).

Since the 18th century, assumptions had been made that this was the source of the Garonne, but the theories were never substantiated. It was not until 1931, that the famous French caver, Norbert Casteret, proved the hypothesis by pouring six barrels of fluorescein into the sink hole. A few hours later, the dyed water came rushing out the other side of the ridge separating Aragon from Catalonia, at Uelhs de Joeu, Val d'Aran. The water had journeyed 3.6km underground.