Each September, millions of monarch butterflies, each weighing only a few grams, begin a journey of up to 2,800 miles from Canada and the Northeast to Transverse Neovolcanic Mountains, 150 miles southwest of Mexico City. The monarch is one of the few insects capable of such a journey (they are capable of trans-Atlantic crossings as well). Amazingly, this migration only takes place every three to five generations, but somehow, by the last week of October, they arrive at the same small groups of oyamel fir trees their ancestors populated the year before.