'Kofun' 古墳 are vast Megalithic tombs consisting of large, keyhole-shaped tumuli, inferred to have been built to serve as Imperial mausoleums. It is thought they were constructed between the early 3rd century and the early 7th century AD.
The Largest of the tombs, the 'Daisen Kofun' (the World's Largest Grave by area) is believed to be the grave of the Emperor Nintoku the 16th Emperor of Japan. It is three-tiered and is surrounded by 3 moats. Measuring 486 meters long, 305 meters wide 'frontage' and about 33 meters in height.
In comparison the base of Great Pyramid of Giza is approx 230 x 230 metres (see diagram).