Find $\tan \left(\frac{2 \pi}{3}\right)$. Enter " $U$ " if it is undefined.

Step 1 ：The problem is to find the value of \(\tan \left(\frac{2 \pi}{3}\right)\).

Step 2 ：The tangent function is defined as the ratio of the sine to the cosine of an angle.

Step 3 ：In the unit circle, the angle \(\frac{2 \pi}{3}\) is in the second quadrant where sine is positive and cosine is negative.

Step 4 ：Therefore, the tangent of this angle will be negative.

Step 5 ：The exact value of \(\tan \left(\frac{2 \pi}{3}\right)\) is approximately -1.732, which is the square root of 3.

Step 6 ：So, the tangent of \(\frac{2 \pi}{3}\) is \(-\sqrt{3}\).

Step 7 ：Final Answer: \(\boxed{-\sqrt{3}}\)