1. The problem is to find the value of $\sqrt{-18}$. 2. The square root of a negative number is not a real number because no real number squared gives a negative result. 3. To handle this, we use the imaginary unit $i$, where $i = \sqrt{-1}$. 4. We rewrite $\sqrt{-18}$ as $\sqrt{18 \times -1} = \sqrt{18} \times \sqrt{-1} = \sqrt{18} \times i$. 5. Simplify $\sqrt{18}$: $\sqrt{18} = \sqrt{9 \times 2} = \sqrt{9} \times \sqrt{2} = 3\sqrt{2}$. 6. Therefore, $\sqrt{-18} = 3\sqrt{2}i$. 7. The final answer is $\boxed{3\sqrt{2}i}$.