The mayor is sending his idea to the City Council as he tries to build momentum for the tunnel, which is more expensive than a rebuilt viaduct, but opens up the land where the structure now sits.

The mayor envisions a series of parks along the waterfront.

"It gives you a sense of the opportunity that we have if we think of this as a people place and not just as a transportation corridor," Nickels said.

The idea has other supporters as well.

"You start with the Olympic Sculpture Park and then have a linear park down to Colman Dock, Seattle is finally getting its downtown park," said Gene Duvernoy of Cascade Land Conservancy.

Opponents said a tunnel is too expensive compared to rebuilding or shoring up the current viaduct.

Those who'd like to see the viaduct gone worry about the impact of years of tunnel construction in this tight space.

"It's basically like a trench that's 60 feet deep and 80 feet wide right in front of our store for four years," said Andy James, who owns Ye Old Curiosity Shop. "It's scary, very scary."

The mayor hopes to put his plans to a public vote in November.