To find the depth of Lake Segundo, we first calculate \( \frac{5}{6} \) of the depth of Lake Profondo, which is 84 feet:
\[ \text{Depth of Lake Segundo} = \frac{5}{6} \times 84 \text{ feet} \]
Calculating this:
\[ \frac{5}{6} \times 84 = \frac{420}{6} = 70 \text{ feet} \]
Thus, the depth of Lake Segundo is 70 feet. Since this elevation is below sea level, we can express the elevation at the bottom of Lake Segundo as:
\[ \text{Elevation at the bottom of Lake Segundo} = -70 \text{ feet} \]
Therefore, the elevation at the bottom of Lake Segundo is -70 feet (below sea level).