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to dig down
to dig into (a matter); to delve into; to probe into; to investigate; to get to the bottom of
考古学者が遺跡を掘り下げている。
Archaeologists are digging down into the ruins.
庭の花壇を作るために土を掘り下げた。
I dug down into the soil to make a flower bed in the garden.
この地層をもっと掘り下げると化石が見つかるかもしれない。
If we dig down further into this stratum, we might find fossils.