tens of thousands of years; many millennia
Used to express an extremely long time span, often in hyperbolic or figurative contexts. Can function as a noun or adverbially.
See also: 万年
This fossil is from tens of thousands of years ago.
A landscape that has remained unchanged for tens of thousands of years stretches out.
Compound of 何 (nan, 'what' or 'how many') + 万 (man, 'ten thousand') + 年 (nen, 'year'). Literally 'how many ten-thousands of years'.