La Piscine is one of the most beautiful museum conversions in Europe: a 1932 Art Deco municipal swimming pool in Roubaix (20 minutes by metro from Lille centre) that has been transformed into a museum of art and industry while retaining the pool at its heart. The Olympic-length swimming pool is still filled with water — artworks are displayed on its pool deck, reflected in the still surface, under the original vaulted skylights that let in extraordinary northern light.
The collection focuses on 19th- and 20th-century French art and design, with particular strength in textiles (appropriate given Roubaix's industrial history as a wool capital), ceramics, sculpture, and drawing. The building itself overwhelms the collection in the best possible way — it is worth coming even if you have no interest in the art. Take the metro from Gare Lille-Flandres to Eurotéléport station, then a short walk through Roubaix's somewhat gritty centre to the museum entrance. Closed Mondays.