Akan-ko is legendary throughout Japan for its marimo (まりも; Cladophora aegagropila), spheres of green algae that are both biologically intriguing – it takes as long as 200 years for them to grow to the size of a baseball – and very, very kawaii (cute). Marimo became endangered after being designated a national treasure: suddenly, everyone in Japan needed to have one. Members of the Ainu community came to the rescue by starting the Marimo Matsuri (まりも祭り), held in early October, which ceremonially returns marimo to Akan-ko.
There are marimo on display at the Akan Kohan Eco-Museum Centre. You can also take a lake cruise out to the Marimo Observation Centre, where a monitor plays live footage of the velvety globes underwater.