This is a link to today’s Google Doodle commemorating the 41st anniversary of the discovery of the mountain of the butterflies.
It was in 1975 that the overwintering place of the monarch butterfly was discovered. The butterflies were found inthe Sierra Madre Mountains in Eastern Mexico. You can read more about how the millions of monarchs cling to the oyamel trees here.
The monarch butterfly is under threat due to climate change and destruction of forests in Mexico, where the species migrates to from the the US and Canada in winter.