Red algal nodule (rhodolith) beach sediments in U-do, South Korea Natural Monument No.438 (<a href="https://ko.wikipedia.org/wiki/%EC%9A%B0%EB%8F%84_%ED%99%8D%EC%A1%B0%EB%8B%A8%EA%B4%B4%ED%95%B4%EB%B9%88" class="extiw" title="ko:우도 홍조단괴해빈">ko:우도 홍조단괴해빈</a>)
<a href="//commons.wikimedia.org/wiki/User:Integral" title="User:Integral">Integral</a>
CC BY-SA 2.0 kr
<span class="int-own-work" lang="en">Own work</span>