Mariposa Monarca

Declared a reserve in 1986 and a UNESCO World Heritage Site in 2008, it protects the oyamel fir forests where the eastern population of the monarch butterfly overwinters after a migration of up to 4,500 km from Canada and the northern United States — a migration unlike any other, because no individual butterfly completes the round trip: several successive generations do it between them, guided by a mechanism science still does not fully understand. Each winter, tens of millions of butterflies literally cover the trunks and branches of the firs, in one of the most spectacular gatherings of migratory insects on the planet, now threatened by illegal logging and climate change.