ancient times; prehistoric times; the distant past; a very long time ago
大昔はこの場所に海があったらしい。
It seems there was a sea here in the distant past.
大昔からこの祭りは続いてきたと聞いた。
I heard that this festival has continued since ancient times.
大昔の人々が残した壁画が洞窟の中で発見された。
Cave paintings left by people from ancient times were discovered inside a cave.