Udo

Udo's Seobin Baeksa beach in Jeju isn't sand at all: the guide explains how its pale, popcorn-shaped grains are rhodoliths, living red algae that pull calcium carbonate from seawater and build up around grains of sand rolling in the surf. It traces how researchers found this to be the only beach in Korea made almost entirely of the nodules, rare enough that officials protected the site as a natural monument in 2004. Look for how the water shifts from turquoise near the shore to a deeper jade further out.