This dried edible seaweed (also called sea mustard, or Wakame in Japanese) is incredibly nutritious and packed full of vitamins and iodine. It’s a key ingredient in seaweed soup (miyeok-guk). It’s sold dried, and when rehydrated it expands quite a bit!
Find it in Korean and Japanese grocery stores or natural foods stores.
Recipes that use dried edible seaweed (mareun-miyeok):