A $250 million drinking water tunnel from Brooklyn to Staten Island that had been delayed by Superstorm Sandy has been activated.

Mayor Bill de Blasio said Saturday that the tunnel is a critical backup that can deliver up to 150 million gallons of safe drinking water per day to Staten Island residents in the aftermath of a disaster.

Called a siphon, the tunnel runs under New York Harbor and replaces two, nearly 100-year-old siphons.

Staten Islanders use about 50 million gallons of water daily that's carries from upstate New York. That will continue to be the main water supply for residents.

Construction on the tunnel was suspended due to damage from Sandy. When work resumed in 2014, new measures were put in to prevent future storm damage.