It records sounds that no human ear can hear, like the low roar of a meteor slicing through the upper atmosphere, or the hum an iceberg makes when smacked by an ocean wave.
It has picked up threats invisible to the human eye, such as the haze of radioactive particles that circled the planet after the Fukushima nuclear accident in Japan in 2011.
The engineers who designed the worldâ€™s first truly planetary surveillance network two decades ago envisioned it as a way to detect illegal nuclear weapons tests. Today, the nearly completed International Monitoring System is proving adept at tasks its inventors never imagined.
The systemâ€™s scores of listening stations continuously eavesdrop on Earth itself, offering clues about man-made and natural disasters as well as a window into some of natureâ€™s most mysterious processes.
The Obama administration hopes the networkâ€™s capabilities will persuade a reluctant Senate to approve a nuclear test-ban treaty that stalled in Congress more than a decade ago. Meanwhile, without the treaty and wholly without fanfare, new stations come on line almost every month.