As the city marks four years since Hurricane Sandy today, Mayor Bill de Blasio officially opened a water tunnel that was delayed because of the storm. NY1's Erin Clarke has that story from Staten Island.

Four-years to the day Hurricane Sandy hit the city, the mayor and others posed for a photo op.

They drank water that came directly from a new $250 million tunnel that will serve as a backup source of drinking water for Staten Island.

In the event of a disaster that might disrupt the water supply, this siphon can transfer up to 150 million gallons of water per day from Brooklyn to the island.

"We have a second source of water for Staten Island that's absolutely protected and will provide the redundancy we need when we're dealing with new kinds of threats," Mayor de Blasio said.

Work on the tunnel had already begun when Sandy hit and its completion was delayed 18 months because of the storm.

That caused the city to go back to the drawing board and come up with a design that officials are now confident is resilient.

While the completion of the water tunnel is a milestone, city officials agree much more needs to be done to get New Yorkers back to where they were before the storm.

Not too far from where the mayor chose to visit on the storm's anniversary, many Staten Island residents are still trying to get back on their feet.

Borough President Jimmy Oddo says that shouldn't be overlooked.

"The mission continues to be to get everyone home," said Staten Island Borough President. "The moment that has been reached, the next mission is to demand an objective assessment of what New York City, what New York State, what the federal government did right and did wrong"

Mayor de Blasio touted accomplishments like elevating mechanical systems in flood prone NYCHA buildings and improvements to area beaches, but admitted the city made many mistakes with the Build it Back Program.

He says by year's end it is expected that work will be started on about 90 percent of the city's 2000 homes severely damaged or destroyed by Sandy, and finished on 75 percent of those.

And that it's been a learning process for city officials who are adjusting to better protect New Yorkers if and when the next natural disaster hits.