Millions of monarch butterflies—enough to cover entire fir forests—migrate every winter from Canada to the mountains of the central highlands of Mexico, creating a swarm of beautiful insects. But there is nothing to fear. These butterflies are remarkably tame and even land all over the hands and heads of visiting tourists.

In 1940, Canadian entomologist Fred Urquhart and his wife Norah Urquhart started a project to learn more about monarch butterfly (Danaus plexippus) migration routes. Helped by many American citizen scientists, they found plenty of wintering sites in the United States. However, the butterflies arriving in Texas seemed to suddenly disappear. A fellow entomologist, Kenneth Brugger, traveled through Mexico, following the monarch butterflies’ migration.

In 1975, he found a spot in the mountains in the state of Michoacan, Mexico, where the butterflies spent the winter months. Since then, those peaks have become a tourist attraction.

Now, to protect the butterflies, this place has become a UNESCO World Heritage Site. The government of Mexico has turned it into a biosphere reserve. During the winter months, all the trees go from green to orange due to the large number of butterflies that live in and on them. The sky looks full of the picturesque creatures.

The reserve is also covered by forests of sacred firs inhabited by a variety of other species of wildlife. But good luck finding them underneath those butterflies.