Environmental activists in kayaks paddled into the middle of Seattle's Elliott Bay on Thursday afternoon to meet -- or, as they say, "un-welcome" -- a huge Shell oil rig.

The arrival of the Polar Pioneer could raise the stakes in the battle over Shell's oil exploration plans in the remote Arctic Ocean.

Seattle City Council member Mike O'Brien said this is a key moment in the fight against drilling in the Arctic.

"If they had other alternatives that were good alternatives, I imagine they would be there, but they're not," he said of Shell Oil. "Yes, I think there is a chance that when Seattle draws the line and says, 'You are not going to use our port,' this may be the straw that breaks the camel's back. This may be the end of Arctic drilling. This fight that's happening in Seattle could be the end of it."

Some people welcomed the rig and the promise of discovering more oil.