To find the number that, when added to \(-70\), gives a sum of zero, you can set up the equation:
\[ x + (-70) = 0 \]
To solve for \(x\), you can add \(70\) to both sides:
\[ x = 70 \]
Therefore, the number that needs to be added to \(-70\) to achieve a sum of zero is \(70\).
The number is 70.