At the end of each North American summer, one of the world’s most astounding migrations begins: Millions upon millions of monarch butterflies make their way from Canada and the United States to an expanse of forest in central Mexico. They settle on oyamel fir trees by the thousands—weighing down branches and blanketing the forest floor. With their arrival, the forest turns a stunning shade of orange.

The 2,800-mile journey takes the butterflies around three months, which makes it the longest insect migration in the world. These monarchs are also the only butterflies known to make a round trip. Around March, they head back north guided by the earth’s magnetic field, the sun’s position, and other internal and external compasses that scientists are still trying to understand.

In February, Atlas Obscura took a handful of travelers to behold Mexico’s Great Monarch Butterfly Migration. Treading silently among the magnificent creatures, Atlas Obscura trip leaders Phil Torres and Becky Friesen guided travelers through the butterflies’ winter habitat: a 200 square mile nature reserve called the Monarch Butterfly Biosphere Reserve.

Lucky for us, Phil also happens to be an amazing photographer. Here’s some of what they saw.