It's an epic journey, one of the longest insect migrations on earth. Each year, millions of monarch butterflies take off from the US and Canada, flying thousands of kilometers to central Mexico in a migration that has given rise to an entire tourist industry.

Monarch Butterflies sit on a tree trunk at the Sierra Chincua Sanctuary, in the mountains of Mexico's Michoacan state, Friday, Dec. 9, 2011.

One by one - in pairs - and in massive teeming clusters they come.

These forests in central Mexico - the singular destination for millions of monarch butterflies.

Here, in the weeks before mating, they pass the days bathing in the sun, or simply resting.

And little wonder.

To reach their winter home, they've flown thousands of kilometers, many starting in Canada, crossing the United States and finally passing through northern Mexico.

It's an astonishing journey, one that just one generation in five is capable of making.

Biologist Katia Lemus said: "This generation is called the Mathusalem generation, because they have a longer lifespan than the others. They can live as long as 9 months."

Lemus helps to monitor the monarch population at the El Rosario reserve.

Lemus said: "Here in the state of México, they find a certain micro-climate, or micro-habitat, that comes with a temperate forest."

Monarch Butterflies sit on a tree trunk at the Sierra Chincua Sanctuary, in the mountains of Mexico's Michoacan state, Friday, Dec. 9, 2011.

The butterflies begin to arrive in late October and early November, often on the same days as Mexico's Day of the Dead holiday.

Audi Gomez, who works at a restaurant serving tourists, says that's lead to certain local legends.

Local resident Audi Gomez Guzman said: "Because they arrive at the same time as the Day of the Dead, the 1st and 2nd of November, people say they are the souls of our dead loved ones coming to visit us."

The clouds of black and orange butterflies can be a breathtaking spectacle, one that draws tens of thousands of tourists each year.

Reporter: "There's no doubt it's worth it, but if you're going to come, come ready for the heights and come ready to climb. By the time you reach the top, you'll be a full three thousand three hundred meters above sea level."

Those heights help to create the environment that has drawn the monarchs for generations.

Scientists aren't sure just what it is that guides them here across the better part of the continent.

But for a few months each year, the monarchs allow us a glimpse at one of the world's most stunning migrations.