SEATTLE - If you fly in or out of SeaTac Airport you already know how busy it is. What you may not know is for the past 15 years a third runway has been in the design and construction phase.

Today an Alaska Airlines 737 performed several touch and go landings on the new air strip as part of the FAA certification process.

The Alaska plane had only four people on board - two of them pilots. And on the ground, a team was busy marking the spot on the runway at the location of tire marks.

"This is basically checking to make sure all the instruments that are on the airfield and on the runway are talking correctly to the aircraft as it comes in to land," said Perry Cooper, Sea-Tac Airport.

That matters, because when this runway opens for regular airline traffic on Nov. 20, the weather could be lousy. Sea-Tac has a reputation for poor visibility, weather that for years forced air traffic controllers to use only one of Sea-Tac's two runways.

"If we get backed up at Sea-Tac that will be a ripple effect at airports across the country," said Cooper. 

Sea-Tac is geographically challenged. At first, they didn't have a place for a third runway. They had to build it with 16 million cubic yards of dirt.

It cost more than a billion dollars to build and has been in the works or more than a decade. 

The project has been steeped in controversy, beginning with the removal of more than 400 homes, as well as apartments and commercial buildings.

Of utmost concern was the environmental impact. Some 20 acres of wetlands were impacted because of the construction. The Port of Seattle had to follow strict federal guidelines to compensate for the loss, including restoration and protection of wetlands and creeks, as well as replacement.