To find the depth of Lake Placid, which is \( \frac{1}{3} \) the depth of Lake Acid, we can calculate it as follows:
- Determine the depth of Lake Acid: 120 ft.
- Calculate \( \frac{1}{3} \) of Lake Acid's depth:
\[ \text{Depth of Lake Placid} = \frac{1}{3} \times 120 , \text{ft} = 40 , \text{ft} \]
Thus, Lake Placid is 40 ft deep below the surface.