Kannonzuka Kofun, Ancient burial mound in Takasaki, Japan.

Kannonzuka Kofun is a keyhole-shaped mound 105 meters long, with a 105-meter-wide, 14-meter-high rectangular front and a 70-meter-diameter, 12-meter-high circular rear. Its horizontal stone chamber runs 15.8 meters, built from megaliths up to 4.5 by 3.4 meters and about 50 tons, earning the name "Gunma's Ishibutai."

This monument dates to the 6th century and remained completely undisturbed until 1945. Workers uncovered the stone chamber while constructing an air raid shelter.

The 15.8-metre burial chamber is built of megaliths, the largest measuring over 4.5 by 3.4 metres and weighing about 50 tonnes; its size has earned it the nickname "Gunma's Ishibutai". Two of the four bronze mirrors found inside were cast from the same mould as mirrors from Inariyama Kofun in Saitama Prefecture. The grave goods are kept at the Kannonzuka Archaeological Museum near the mound.

The site is easiest reached from Takasaki or Gunma Hachiman station, followed by a short walk. Access is on foot and visitors can view the structure from outside.

The stone chamber was built with considerable craftsmanship and held about 300 artifacts including bronze mirrors and weapons. This abundance of objects reveals the technical skills and lifestyle of that era.