Lac d'Ilay, Glacial lake in Jura, France.

Lac d'Ilay is a glacial lake in the Jura, 1.9 km long and 400 m wide, lying at 774 m on the Champagnole plateau. Its northern basin reaches 32 m deep, while the southern part is shallow and bordered by reeds. A small rocky island, Île de la Motte, lies 200 m from the northern shore.

The lake lies in a basin lined with glacial marl and moraine deposits from the Jura glaciers, which let water stand on karstic ground. On the Île de la Motte, 200 m from the north shore, Saint Desiderius founded the Benedictine priory of Saint-Vincent in the 6th century; remains from Charlemagne's time (around 800-850) were uncovered there. The monks cleared the land, and the priory was destroyed in the 17th century.

The lake's other name, Lac de la Motte, comes from the small rocky Île de la Motte, 200 metres off the north shore. There, according to the German source, Saint Desiderius founded the Priory of Saint-Vincent in the 6th century; the monks cleared the land, laying the basis for the hamlets and villages around. The priory was destroyed in the 17th century.

The lake is accessible via a walking path that offers several viewpoints and access points to the water. Visitors should note that certain shoreline areas are protected and off-limits to preserve the ecosystem.

The lake's name "Ilay" comes from the Île de la Motte, a rocky island 200 m off the north shore where the Benedictine priory of Saint-Vincent-de-la-Motte was built in the early Middle Ages; remains from Charlemagne's era (around 800-850) were uncovered there. The priory was destroyed in the 17th century.