Last year, 1.4 billion people traveled the world. That’s up from just 25 million in 1950. In China alone, overseas trips have risen from 10 million to 150 million in less than two decades.

This dramatic surge in mass tourism can be attributed to the emergence of the global middle class, and in some ways, it’s a good thing. But the consequences are grave—particularly for the planet. In a new episode of The Idea File, the staff writer Annie Lowrey explains how overtourism has contributed to large-scale environmental degradation, dangerous conditions, and the immiseration and pricing-out of locals.

“Tourists can alter the experience of visiting something such that they ruin the very experience that they’ve been trying to have,” Lowrey says in the video. “That’s the essential definition of overtourism.”

For more, read Lowrey’s article, “Too Many People Want to Travel.”