By the sacred fire at the heart of the Oceti Sakowin camp, a woman swaddled in layers of clothing makes announcements over a small loudspeaker. “We have a missing woman,” she calls. “Has anyone seen a woman: tall, blonde hair, a blue poncho, and no shoes?” The camp is here, on the border of the Standing Rock Sioux Reservation in North Dakota, to oppose the Dakota Access oil pipeline. But right now the priority is keeping people alive. The temperature is minus 14 centigrade and several inches of snow have fallen. Exposed skin can suffer frostbite in 30 minutes. The missing woman is soon found, but she is one of many people at the camp unprepared for the brutal North Dakota winter. Thousands of environmental activists from the US and around the world have come to support the Native Americans’ battle against the pipeline, many mobilised by the campaign’s popular Facebook pages. The pipeline’s route, which passes the camp about half a mile away, is intended to run under Lake Oahe on the Missouri river, raising fears about possible spills into the water used by the Standing Rock tribe.