RA 138 is a military bunker located near Saint-Cast-le-Guildo in the Côtes-d'Armor department of Brittany, France. This coastal defense structure was part of France's Atlantic Wall fortifications during World War II, built by German forces occupying the region. The bunker's position near the English Channel made it strategically important for coastal defense and observation.
Today, it stands as a historical reminder of the extensive military infrastructure constructed along France's northern and western coasts during the war.