To find the depth of Lake Segundo, which is \(\frac{5}{6}\) as deep as Lake Profondo (which has a depth of 84 feet), we can calculate it as follows:
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Calculate the depth of Lake Segundo: \[ \text{Depth of Lake Segundo} = \frac{5}{6} \times 84 \text{ feet} \]
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Perform the multiplication: \[ \frac{5 \times 84}{6} = \frac{420}{6} = 70 \text{ feet} \]
So, Lake Segundo is 70 feet deep.
Since we need to express the elevation at the bottom of Lake Segundo (below sea level), we represent it as a negative value. Therefore, the elevation at the bottom of Lake Segundo is:
\[ \text{Elevation} = -70 \text{ feet} \]
Thus, the elevation at the bottom of Lake Segundo is \(\boxed{-70}\) feet.