Story highlights Six protesters scale the Polar Pioneer, hundreds of miles northwest of Hawaii

Greenpeace opposes Shell's plans to drill for oil in the Arctic

(CNN) Greenpeace activists have climbed aboard a Shell oil rig to protest the company's plans to drill in the Arctic near Alaska.

The six protesters used ropes and harnesses Monday to scale the huge platform in the Pacific Ocean, tweeting images of their daunting climb as they went.

"We made it! We're on Shell's platform. And we're not alone," wrote Aliyah Field, an American activist taking part in the protest. "Everyone can help turn this into a platform for people power!"

The rig, the Polar Pioneer, is on its way to the Arctic via Seattle. The environmental activists caught up with it about 750 miles northwest of Hawaii, Greenpeace said

From a distance it looked so small... #TheCrossing pic.twitter.com/V45isnawNl — Andreas Widlund (@WidlundAndreas) April 6, 2015

They plan to occupy the underside of the rig's main deck and say they have enough supplies to stay there for several days.

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