Early morning in the mountains of Michoacán. There’s a stillness in the wooded glades and a delicate scent of piny resin in the air. Mostly oyamel firs, the trees are oddly coated in a scrunched orange blanket – some kind of fungus? Diseased bark? Then the sun breaks through the mist and thousands of butterflies swoop from the branches to bathe in the sunlight, their patterned orange and black wings looking like stained-glass windows or Turkish rugs – the original Mexican wave. The forest floor is carpeted with them. Branches buckle and snap under their weight. And there’s a faint noise, a pitter-patter like gentle rain – the rarely heard sound of massed butterflies flapping their wings.

The annual migration of hundreds of millions of monarch butterflies from North America to this small area of central Mexico – no more than 96 square kilometres – is one of the last mysteries of the scientific world. For years, their winter home was known only to the locals, but in 1975, two determined American biologists finally pinpointed the location, and now visitors (mainly Mexican) flock here during the season to witness one of nature’s most impressive spectacles. In the silence of the forest sanctuary, people stand stock-still for hours at a time, almost afraid to breathe as millions of butterflies fill the air, brushing delicately against faces and alighting briefly on hands.

No one is entirely clear why the butterflies have chosen this area. Some say it’s the oyamel’s needle-like leaves, ideal for the monarch’s hooked legs to cling onto; or that the cool highland climate slows down their metabolism and allows them to rest and lay down fat before their arduous mating season. The Aztecs, however, had other ideas, believing that the butterflies – which arrive in Mexico shortly after the Day of the Dead on November 1 – were the returning souls of their fallen warriors, clad in the bright colours of battle.

The best place to see the butterflies is in the butterfly sanctuary near the village of El Rosario (mid-Nov to late March daily 9am–4pm; www.mariposamonarca.semarnat.gob.mx).