Daisenryo Kofun: Japan's keyhole shaped tombs explained

Distinctly shaped ancient burial grounds known as "kofun" are found all across Japan. Almost identical to the shape of a keyhole, the structures are ancient burial mounds built thousands of years ago. Kofun range from several metres to over 400m long, with the largest - the Daisen Kofun - having been attributed to Emperor Nintoku, in Sakai City, Osaka.