To simplify \((6^3)^5\), we can use the power of a power rule, which states:
\[ (a^m)^n = a^{m \cdot n} \]
Applying this rule:
\[ (6^3)^5 = 6^{3 \cdot 5} \]
Now, calculate \(3 \cdot 5\):
\[ 3 \cdot 5 = 15 \]
So, we have:
\[ (6^3)^5 = 6^{15} \]
Thus, the simplified expression is:
\[ \boxed{6^{15}} \]