Tsunamis are infrequent but terrifying hazards for coastal communities. Difficult to predict, they materialize with little warning, claiming thousands of lives and causing billions of dollars in damage. Developing countries cannot afford costly underwater cable systems, and governments and relief organizations have been forced to rely on flawed warning systems such as deep-sea buoys. Now, a groundbreaking new approach to tsunami detection and warning, which relies on low-cost underwater sensors and networks of smartphone communication, has changed the equation.