Earthquakes pose a constant threat to communities worldwide, and providing people with a few seconds of warning before the shaking starts can be lifesaving. The goal of earthquake early warning (EEW) systems is to give people precious seconds to take cover, but these systems are often expensive and limited to certain regions. Researchers have developed an Android Earthquake Alerts system that uses the global network of Android smartphones as a powerful earthquake detection system. The system has detected thousands of earthquakes and sent alerts to millions of people in nearly 100 countries, often providing crucial seconds of warning. The system works by using the accelerometer in Android phones to detect the initial P-wave of an earthquake, which is then analyzed to confirm the earthquake and estimate its location and magnitude. The system sends out two types of alerts: BeAware for light shaking and TakeAction for stronger shaking. The system has been rolled out in 98 countries and has detected over 18,000 earthquakes, sending out 790 million alerts worldwide. The system has improved its magnitude estimation over time, with a median absolute error of 0.25, and has received overwhelmingly positive feedback from users. The system is constantly learning and improving, and its data is helping to build more accurate prediction models, with the potential to deliver rapid post-earthquake information to emergency responders in the future.