Most schoolchildren in the United States have a class on monarch butterflies. I went to school someplace that wasn’t really in the middle of their migration paths, but even so we had a whole thing on them, raising a few in a terrarium and seeing the whole caterpillar/chrysalis/butterfly lifecycle. I suppose it’s different in the South, or along one of the main migration routes, but I never actually got a sense they were going anywhere. You’d see a few around every spring and autumn, and then they’d just kind of go away a couple weeks later.

But they were migrating, and the vast, vast majority of them — hundreds of millions — returned to just twelve sites in the mountains of Mexico to spend the winter. And you can visit them, over the winter. You imagine it’s going to be like an open air butterfly house, but it’s not. First, there are hundreds and thousands of them. Second, they’re not just aimlessly fluttering around. They’re sweeping past en masse in this unceasing flood of monarch butterflies, so many it feels like it’s hard to breathe, just from the sheer numbers of them.

One of the impediments towards saving the planet is that humans don’t live on the timescales necessary to notice the damage we’re doing. The monarchs are struggling, between the pesticides and the loss of milkweed, and it’s only when you get someplace like Cerro Pelón that you get to understand what it used to be like. And standing there, overwhelmed by the flash of wings, feeling thousands of them gently brush past you, will drive that home stronger than anything else possibly could.