To simplify the expression \( (78^8)^6 \) using the Power Rule of Exponents, which states that \( (a^m)^n = a^{m \cdot n} \), we can apply this rule as follows:
\[ (78^8)^6 = 78^{8 \cdot 6} \]
Now, calculate \( 8 \cdot 6 \):
\[ 8 \cdot 6 = 48 \]
So, we have:
\[ (78^8)^6 = 78^{48} \]
Thus, the simplified expression is:
\[ \boxed{78^{48}} \]