To find the depth of Lake Segundo, we need to calculate 5/6 of the depth of Lake Profondo.
First, we find 5/6 of 84 feet:
\[ \text{Depth of Lake Segundo} = \frac{5}{6} \times 84 \text{ feet} = 70 \text{ feet} \]
Now that we have the depth of Lake Segundo, we can express the elevation at the bottom of Lake Segundo below sea level. Since depths are below sea level, we represent this as a negative value.
Thus, the elevation at the bottom of Lake Segundo is:
\[ \text{Elevation} = -70 \text{ feet} \]
Therefore, the elevation at the bottom of Lake Segundo is -70 feet.