Japan's tallest dam at 186 m, wedged into a gorge in the Northern Alps and reached through a tunnel by electric bus. From late June to mid October it releases water in a plume you can feel from the walkway. Be honest about the journey: roughly two hours each way via Shinano-Omachi and Ogizawa, so this is a full day rather than an afternoon. It is also the eastern gateway to the Tateyama Kurobe Alpine Route.