Marimo (Aegagropua linnaei) that grows in Lake Akan was discovered over one century ago and it is designated as a special natural monument of Japan for its rareness. Marimo is an aggregate of thin plant fibers, forming a round ball shape. Its ecology has not been completely understood. Colonies of Marimo exist in Lake Akan and Lake Mývatn in Iceland, however, only the ones in Lake Akan are large in size and neatly spherical in shape. From April through to November natural Marimo can be observed at the Marimo Exhibition and Observation Center located on Churui Island of Lake Akan which can be reached via a cruise boat.