米の生産量を格段に上げるために大規模な開墾を行えば、必然的に大量の土が出ます。
When bringing their land under dynamic cultivation to dramatically increase their rice production, they had to do something with the mounds of leftover soil.
そして、大量の水の供給が必要となるため、大きな河川の近くに広大な水田が作られたことでしょう。
Plus, to accommodate the need of obtaining a great amount of water supply, they must have made a vast land of rice paddies in the vicinity of major rivers.
そこで、たびたび起こる洪水などからその広大な水田や人々の住む地域を守れるような巨大な堤防が不可欠となり、当時の豪族たちは開墾で出た土を活用して大きな古墳を作っていたものと思われます。
In need of protecting their vast land of rice paddies and residential area from often cases of river floods, they wisely used the mounds of leftover soil to make huge mounted tombs which worked as huge banks.
ゆえに、もはや大規模な開墾や治水の必要性がなくなる7世紀頃までに全国でおよそ16万基近くもの巨大な古墳が作られていたのでしょう。
This is why, the powerful families throughout Japan had built nearly 160,000 huge mounted tombs up to around the 7th Century when major cultivation and irrigation were no longer necessary.
もっとも、中には海外視察団や他の豪族たちに力を誇示する目的で作られたものもあると思います。
Some of them were built to ostentatiously flaunt their power to the missions from overseas and to the other powerful families in Japan.
