Banc d'Arguin Natural Reserve, Natural reserve at entrance of Arcachon Bay, France.

Banc d'Arguin is a sandbank about 4 km long and 2 km wide at low tide, lying between the Dune du Pilat and the Pointe du Cap Ferret at the entrance to Arcachon Bay. Its pale beige sand forms a long crescent, with darker rippled bands and damp patches along the inner edge, a small pool enclosed by a narrower strip of sand, and an outer edge dropping steeply into dark blue-green water. Currents, tides and wind continually reshape it.

The sandbank formed over roughly 20,000 years as littoral drift, swell, wind and currents moved sediment south along the Aquitaine coast; the mouth of the Arcachon basin shifted from west to south-west, leaving a mobile sandy islet from Cap Ferret material. The bank was classified a national nature reserve in 1972, and a decree of 10 May 2017 replaced the 1986 creation decree with a 4,360-hectare perimeter.

The name Arguin may come from Gascon "ar guin", meaning "offshore"; Cassini's 18th-century maps already marked the bank, then closer to Cap Ferret. A temporary structure on the emerged sand holds an exhibition on the moving island and the cycle of the Arcachon passes, staffed by reserve personnel in summer. Access to nesting zones is closed, and bivouac and domestic animals are banned.

The site is accessible only by boat from either Lège-Cap Ferret or Arcachon, with crossing times between 20 and 60 minutes. Check tide schedules before your trip, as they determine which areas of the sandbanks are visible and walkable.

The sandbanks continuously shift through wind and ocean currents, meaning the landscape is never the same twice. Each visit reveals a different configuration of islands and new nesting sites for the birds.