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In eastern Hokkaido there is a lake called Lake Akan, where the marimo grow. A marimo is a rare growth form of the filamentous green algae (Chlorophyta), where the algae grow into large green balls with a velvety appearance.
The Ainu, northern Japan's indigenous people, called them lake goblins or lake rollers. It turns out that the only other regions where colonies of marimo are known to form are Iceland, Scotland, and Estonia, but the ones in Lake Akan are known to grow particularly large-- up to 30 cm across.
When I was young, we used to think of marimo as a sort of pet, and kept them in jars filled with water. I was a lazy guardian, and my marimo invariably died, but, if memory serves, one of my sisters kept one alive for a long time, growing it to an impressive size. Nowadays they are a protected species, and the little ones you find in souvenir shops are hand rolled from free-floating filaments.