The 56,259 ha bios­phere lies with­in rugged forest­ed moun­tains about 100 km north­west of Mex­i­co City. Every autumn, mil­lions, per­haps a bil­lion, but­ter­flies from wide areas of North Amer­i­ca return to the site and clus­ter on small areas of the for­est reserve, colour­ing its trees orange and lit­er­al­ly bend­ing their branch­es under their col­lec­tive weight. In the spring, these but­ter­flies begin an 8 month migra­tion that takes them all the way to East­ern Cana­da and back, dur­ing which time four suc­ces­sive gen­er­a­tions are born and die. How they find their way back to their over­win­ter­ing site remains a mys­tery.