Étang de Saint-Quentin, artificial pond in Saint-Quentin-en-Yvelines, Yvelines, France

The Étang de Saint-Quentin is an artificial pond of about 150 hectares in Trappes and Montigny-le-Bretonneux, in the Yvelines, the largest body of water in Île-de-France. A long low wall of grey stone blocks holds back the water, with a clipped hedge and tall trees behind it; a small square stone hut with a pitched brown roof stands at the water's edge. Reeds with brown seed heads grow in the shallows, and an 87-hectare national nature reserve occupies the western part.

The Étang de Saint-Quentin was created in the 17th century as part of a large hydraulic project to supply water to Versailles fountains. Over centuries, the site experienced changes, including damage from World War II that affected water flow and depth.

The pond takes its name from a chapel dedicated to Saint Quentin, demolished in 1780, which was said to have held the saint's relics; it was long called the étang de Trappes. Until 1914, residents of Montigny-le-Bretonneux, mostly day labourers and farm workers, wove rushes from the pond's osier bed into mats and doormats. Around the water, royal boundary stones carved with a fleur-de-lis in relief, some defaced during the Revolution, still mark the Versailles water network.

The pond lies in the Yvelines, in the new town of Saint-Quentin-en-Yvelines, shared by Trappes and Montigny-le-Bretonneux (Pas-du-lac district). The 87-hectare national nature reserve created in 1986 occupies the western part, where access is restricted; the leisure base on the shore gives onto the water. The Trappes aqueduct, cut in 1978, no longer links the pond to Versailles.

Around the pond stand royal boundary stones of grey sandstone, 80 cm to 1 m tall, carved from the early 18th century with a fleur-de-lis in relief inside an oval; some bear a royal crown, others were defaced or covered by revolutionaries. About 200 of the original thousand remain.