A monarch butterfly sanctuary in central Mexico opens to visitors, showcasing millions of butterflies clustered on oyamel fir trees. | Reuters

Each fall, monarch butterflies embark on one of nature’s greatest migrations, flying thousands of miles to the oyamel fir forests of central Mexico where they spend the winter in massive colonies covering tree trunks and branches. Along the way, they stop to feed on late-blooming wildflowers and gather in overnight roosts. These clusters protect them from cool nights, conserve energy for the next day’s flight, and provide safety in numbers. The Central Platte River Valley in Nebraska is one of t