In parshas Bechukosai Vayikra (26:33) the torah fortells of the Babylonian exile. The Jews returned to the land of Eretz Israel from exile in Egypt in the year 2488 From Creation (F.C.). The Jews remained in Eretz Israel until the year 3338 F.C. when the Temple was destroyed and the Jews were sent into exile in Babylonia. The Jews returned from exile in Babylonia to build the Second Temple in the year 3408 F.C.
How long were the Jews in exile in Bablyonia?
3408 – 3338 = 70 years