70

70 = 2 x 5 x 7.

The sum of its divisors, including 70 itself, is a square (144): 1 + 2 + 5 + 7 + 10 + 14 + 35 + 70 = 144 = 12².

70 has a unique representation as a sum of three squares: 70 = 3² + 5² + 6².

70 is the 5th pentatope number.

70 is 77 in base 9.

Three score years and ten (70) is traditionally considered the natural human life span.

Source: The Penguin Dictionary of Curious and Interesting Numbers, rev. ed., by David Wells.