Saint-Omer (French pronunciation: [sɛ̃.t‿omɛʁ]; West Flemish: Sint-Omaars) is a commune and sub-prefecture of the Pas-de-Calais department in France. It is 68 km (42 mi) west-northwest of Lille on the railway to Calais, and is located in the Artois province. The town is named after Saint Audomar, who brought Christianity to the area. The canalised section of the river Aa begins at Saint-Omer, reaching the North Sea at Gravelines in northern France. Below its walls, the Aa connects with the Neufossé Canal, which ends at the river Lys.