An observation deck on the 21st floor of Osaka's Sakai City Hall provides visitors with a bird's-eye view of the surrounding area. In the midst of the urban jungle a few kilometers away, a large patch of greenery in the distance looks out of place amid the sea of modern buildings, like a public park that managed to escape development.

Up close, however, the green oasis turns out to be not a park but the burial mound, or kofun, of Emperor Nintoku. Officially known as Daisen Kofun, it's the centerpiece of what is known as the Mozu-Furuichi Kofun Group.

After years of lobbying efforts by Osaka Prefecture and the central government, UNESCO announced in July that 49 kofun in the ancient group in and around Sakai would be added to its World Heritage list.