SEATTLE -- The New Arena at Seattle Center, which will be home to the Seattle NHL expansion franchise, is on schedule to open in the summer of 2021.

The $930 million project, overseen by Oak View Group, includes keeping the roof of the building, formerly named Key Arena, in place while the construction is done underneath.

"This is a brand-new arena being built," Ken Johnsen of Oak View said Monday during a media tour of the construction site. "The old arena is gone. There's really nothing left except for that roof. We're holding up that roof temporarily and we're digging down so we can build a brand-new arena that's the best in the country."

The old arena grandstands have been removed, and with the windows gone the public has a clear look through the structure. All that remains under the roof are mounds of dirt along the outside of the building. In the center of the floor, a giant beam reaches up to the center of the roof to provide support.

It's one of 70 columns the construction team has added around the building to support the roof, which weighs 22,000 tons. The old columns are suspended in the air by the temporary columns and will stay propped up in that manner until crews build back up during 2020.

Having been designated a historic landmark, the roof, which has been part of the Seattle skyline since the building opened for the 1962 World's Fair, had to stay, giving the project an extra set of challenges and costs.

Johnsen said the beams are holding the roof securely in place and that it hasn't moved or shifted since construction began at the beginning of 2019.

Underneath the roof has been a buzz of activity, with crews and machinery working around the clock as they prepare to dig down 15 feet from the current floor.

"We've got 120 people working on site right now, and it will probably peak over 1,000 in a little over a year," Johnsen said. "We'll start hauling those mounds at the side by the end of the month, all the dirt excavated by end of year."

When finished, more than 600,000 cubic yards of dirt will have been removed, which will expand the arena's size from 400,000 square feet to 800,000.

Once the dirt has been removed and the crews have dug down, they'll begin building back up. That build-up includes reconnecting the suspended columns with new support structures to make the roof part of the new building.

Seattle was awarded an expansion franchise by the NHL in December 2018 and is slated to begin play for the 2021-22 season. The Seattle ownership group has hinted that it would like to host the 2021 NHL Draft, which would mean the arena would need to be completed by June 2021.

Johnsen said the new building is on target to open in the summer of 2021, but he can't set an exact date yet.

"We're going to set that in the early spring," Johnsen said. "There's work to be done to get all the way to the bottom. Once in early spring we see where we are, and if everything is on track, we can set a target date."

The completed arena will house the new NHL team and be a concert venue. The Seattle Storm of the WNBA also will play in it.