I’ve taken people to see this kofun (burial mound) in Yamaga several times. It’s worth seeing, I think. The following page has a link to a map showing the location:
http://www.tvtrip.com/Monument+1-info/Chibusan-Kofun+u1HcZm
Here’s an old message from Kumamoto-i with some more information:
http://groups.yahoo.com/group/kumamoto-i/message/7875
There’s also more explanation in English toward the end of the text explanation that accompanies this YouTube video.