The Porcupine caribou herd grazes near Beaufort Lagoon in Alaska’s Arctic National Wildlife Refuge. Every spring, this 165,000-member herd, named for the Porcupine River which runs through its winter grounds, migrates 400 miles (644 kilometers) over the Brooks Range to calving grounds on the coastal plain.

(Text adapted from and photograph shot on assignment for, but not published in, the National Geographic book America’s Hidden Wilderness; Lands of Seclusion, 1988)