To find the length of the diagonal of a square, we can use the formula:
\[ \text{Diagonal} = s \times \sqrt{2} \]
where \( s \) is the side length of the square.
Given that the side length \( s \) is 700 feet, we can substitute this value into the formula:
\[ \text{Diagonal} = 700 \times \sqrt{2} \]
Using \( \sqrt{2} \approx 1.4 \):
\[ \text{Diagonal} \approx 700 \times 1.4 \]
Now, perform the multiplication:
\[ 700 \times 1.4 = 980 \]
Thus, the approximate length from one corner to the opposite corner of the square park is:
\[ \boxed{980} \text{ feet} \]