BIRKENSTOCK. 1010370. COTTON SOLE UNDERCOVER SOCK. BLUE. The sporty sneaker socks are almost invisible thanks to their minimal shape. The Cotton Sole sock boasts a perfect base thanks to its integrated Birkenstock Footbed Comfort Sole. A special reinforced section in the hand-looped toe area enables a perfect fit which is further supported by the comfy elasticated ribbed cuff. The outstanding comfort is rounded off by sustainable high-quality organic cotton.