Officials are calling the project the McKinley Water Vault, with the tank 30 to 40 feet underground -- spanning an acre and a half.
During heavy storms, the tank would collect millions of gallons of waste water and storm water. That water will flow back into the sewage system.
The city says the project will take years to complete, and engineers are looking for ways to prevent foul odors.
Neighbors are concerned the project's long timeline will keep them from enjoying the park's beauty.
Money from water rate increases will pay for the water vault.