To find the volume of a pyramid, we can use the formula:
\[ \text{Volume} = \frac{1}{3} \times \text{Base Area} \times \text{Height} \]
In this case, the base is a rectangle, so we first find the area of the base:
\[ \text{Base Area} = \text{length} \times \text{width} = 50, \text{ft} \times 20, \text{ft} = 1000, \text{ft}^2 \]
The height of the pyramid is the depth of the lagoon, which is 25 feet.
Now we can calculate the volume:
\[ \text{Volume} = \frac{1}{3} \times 1000, \text{ft}^2 \times 25, \text{ft} \]
\[ \text{Volume} = \frac{1}{3} \times 25000, \text{ft}^3 = 8333.33, \text{ft}^3 \]
So the lagoon holds 8,333.33 feet cubed (or 8,333.33 ft³).
The answer is:
8,333.33 ft.³