To find the area of a square, you can use the formula:
\[ \text{Area} = \text{side} \times \text{side} \]
In this case, each side of the baseball infield is 30 yards long. So the area is:
\[ \text{Area} = 30 , \text{yards} \times 30 , \text{yards} = 900 , \text{square yards} \]
Therefore, the area of the baseball infield is 900 square yards.