From the San Diego Union Tribune:
Near the top of Twin Oaks Valley Road, workers are completing what is believed to be the largest concrete water tank of its type in the world.
To get an idea: Imagine the infield of Qualcomm Stadium when the Padres played there, and look up to the Plaza Level seats 40 feet above the field.
A massive water tank is being built by the Vallecitos Water District.
Capacity: 40 million gallons
Dimensions: 40 feet high, 432 feet in diameter
Materials used: 22,950 cubic yards of concrete, 575,000 pounds of steel
Cost: $21 million