In the late 1970s, biologists studying the cloud forest had a problem: the most interesting plants and animals lived high up in the canopy, but climbing every tree was slow and dangerous. So they strung cables between the tall trees and learned to slide along them.
Soon, every scientist in the forest had a pulley in their backpack. They could whizz from one giant tree to the next, taking notes on flowers and watching birds at branch level. It was much faster than walking.
Children visiting the forest spotted the scientists doing this and thought it looked amazing. So did the local guides. Within a few years, zip-lining became a way for visitors to feel like a bird gliding through the trees. Now you can do it almost anywhere in the world.
Modern zip-lines in Costa Rica are sometimes a kilometre long, and you can travel at over 70 kilometres an hour. The longest ones cross deep forest valleys, with nothing but treetops below your dangling feet.