💨 What happened when the Earth had 35% oxygen?

What would happen to the planet if oxygen levels rose from the usual 21% to as much as 35%? Dragonflies the size of hawks, millipedes the length of crocodiles and vast forests that changed the very atmosphere of the Earth - this is not a fantasy, but a reality that existed 300 million years ago. Why did this ancient world disappear - and what lessons does it hold for today's climate?