A replica of an ancient boat set off Sunday on a monthlong experimental voyage from Kumamoto to Osaka in a bid to resolve an archaeological mystery -- how heavy stone coffins were shipped in the sixth to seventh centuries to emperors' tombs in the Kinki region some 800 km away.

The 12-meter-long wooden boat, rowed by 18 people, set off in the Ariake Sea from Uto, Kumamoto Prefecture, towing rafts carrying a "makadoishi" stone coffin and lid.

The stone is volcanic tuff created by the eruption of Mount Aso in Kumamoto Prefecture 80,000 to 90,000 years ago.

The pinkish stone is unique to areas of Kumamoto, and has been found as material for coffins in the burial mounds of ancient emperors and local rulers in the Kinki region. The region is around current Osaka and where Japan's early city states were formed.

A group of archaeologists and the city of Uto are carrying out the voyage project. They hope they can find out how the stone coffins were shipped.

Escorted by three motor vessels, the boat and coffin are expected to stop at 22 ports through the Seto Inland Sea and arrive at a port in Osaka Bay around Aug. 26.