It's hard to discern, but Horinouchi Kaizuka Shell Mound is as long as 225 meters and 100 meters wide.

Pink portion in the diagram on the upper left shows where the shells, animal bones (mostly boar and deer) and other food waste were tossed on the hillsides during 4,000 to 2,500 years ago (Jomon Period). They form a horseshoe shape on the hill.
The mound was a central feature of the Jomon Period cluster of homes in this area. Since the late 19th century (Meiji Period), many digs and research on this shell mound have been conducted by archaeologists. However, there are still unknown things about mound.