1. The problem is to find the square root of $-254$. 2. Recall that the square root of a negative number involves imaginary numbers because no real number squared gives a negative result. 3. The formula for the square root of a negative number $-a$ (where $a > 0$) is $\sqrt{-a} = i\sqrt{a}$, where $i$ is the imaginary unit with the property $i^2 = -1$. 4. Applying this to $-254$, we have: $$\sqrt{-254} = i\sqrt{254}$$ 5. Since $254$ is not a perfect square, we leave it under the square root. 6. Therefore, the square root of $-254$ is: $$\boxed{i\sqrt{254}}$$