A monarch butterfly is a large orange and black butterfly famous for one of nature's most remarkable journeys. Every fall, millions of monarchs migrate up to 3,000 miles from Canada and the United States to spend winter in the mountains of central Mexico or along the coast of California. They return north in spring, but no single butterfly makes the whole round trip: the journey takes multiple generations, with great-great-grandchildren completing what their ancestors started.

Monarchs are easy to recognize by their bright orange wings with black borders and white spots. These vivid colors warn predators that monarchs taste terrible because, as caterpillars, they eat milkweed plants that contain bitter toxins. The toxins don't hurt the monarchs but make them poisonous to birds and other animals.

Scientists still don't fully understand how monarchs navigate across entire continents to places they've never been. Somehow, using the sun's position and Earth's magnetic field, they find their way to the same forests their great-great-grandparents used the year before. In Mexico, so many monarchs cluster on the trees that their weight bends the branches, creating what looks like orange waterfalls.

Monarchs face serious threats from habitat loss and climate change, making their epic migration increasingly difficult.