Lac de Bastan inférieur, Glacial lake in Hautes-Pyrénées, France.

Lac de Bastan Inférieur is a glacial lake in the Arbizon massif of the Pyrenees, within the commune of Vielle-Aure in the Aure valley. It lies at 2,141 m, covers 1.8 ha and reaches 13 m deep. It is flanked by the pic de Bastan and pic de Bastan d'Aulon and overlooks the lac de l'Oule just below; the refuge de Bastan stands nearby.

The lower Bastan lake is a glacial lake in the Arbizon massif, one of three Bastan lakes that form a chain of glacial lakes. It lies at 2,141 m, covers 1.8 ha and is 13 m deep. It is fed by the upper and middle Bastan lakes, and the Bastan stream flows out of it into the lake de l'Oule below.

Bastan is a generic term for the valley where the lake lies, not a name specific to this one lake. The three Bastan lakes — upper, middle and lower — are described in the source as forming a chain of glacial lakes, and the stream of Bastan flows out of the lower lake into the Lac de l'Oule just below.

The lake can be reached by a marked trail from the Artigusse parking area at about 1590 meters, following the GR 10 path through the nature reserve. The climb takes several hours and is best done in dry conditions since the ground can become slippery.

The three Bastan lakes form a chain of glacial tarns, and the lower one is by far the smallest at barely 1.8 hectares, against 13 metres of depth. Its outflow, the ruisseau de Bastan, runs down into the lac de l'Oule just below. The Bastan refuge stands close by.