Listening practice: The network that listens to Earth

Imagine that the Earth could whisper. A wave moves through rock. A pressure signal crosses the deep ocean. A sound too low for human ears spreads through the sky. Somewhere else, a machine pulls air through a filter, looking for particles so tiny that nobody can see them. According to the Comprehensive Nuclear-Test-Ban Treaty Organization, or C T B T O, these signals are part of a global system built to watch for nuclear test explosions. The idea is not to look with one giant camera. It is to listen with many different kinds of ears.