Marimo (毬藻; also known as “moss balls”) is a type of algae that grows spherically as it rolls in the water. It only grows 5mm a month and is easy to take care for, but it is very rare and can only be found in a few parts of the world (Japan, Iceland, Scotland, and Estonia).
To celebrate and conserve this rare and beautiful species, a three-day festival called Marimo Matsuri takes place every year in Hokkaido, Japan. All the marimo plants in the Lake Akanko are taken out, individually cleaned, and placed back into the water. (via)