Agde Round Lock, Canal lock in Agde, France.

The Agde Round Lock is a lock on the Canal du Midi in Agde, built in 1676 of black basaltic stone. It originally had a circular chamber 29.20 m in diameter and 5.20 m deep, with three sets of gates opening onto different water levels: one toward Béziers, one toward the Étang de Thau, and one toward Agde. The chamber was lengthened after 1978 to accommodate barges up to 38.50 m long, so it is no longer round.

The lock was built in 1676 from volcanic stone as a solution for managing different water levels between the canals. Later modifications in the 1970s adapted it to accommodate larger vessels, supporting the region's growing trade.

The lock's three gates each open onto a different water level: boats rise toward the Étang de Thau and descend toward Agde. Around it once stood an administration building, stables, shops and a chapel. A second French round lock, the now-disused Écluse des Lorraines on the Canal latéral à la Loire, links that canal with the Allier.

You can view the lock from the shore and watch boats going through while changing water levels. It is best to visit during the day when boats regularly pass through and you can see the lock working.

The lock's three gates each open onto a different water level: boats rise towards the Étang de Thau and descend towards Agde. Its round chamber let crews turn to pick an exit, and a second French round lock, the now-disused Écluse des Lorraines on the Canal latéral à la Loire, is the only counterpart.