Immemorial
-going deep into the past before history, knowledge, memory.
synonyms: prehistoric, archaic, ancient
antonyms: modern, current, decade
sentences:
Antarctic layers of ice can be excavated to study the climates of immemorial times nearly 160,000 years ago.
The Historian specialized in the Immemorial periods of history when archaic civilizations existed such as ancient Babylon.
Ruins of Ancient City of Babylon, Iraq 1932