A shutdown of a nuclear reactor in southern Florida on February 26 led to a cascade effect, leading to blackouts across the region, cutting off power to 3 million people, and forcing hospitals and airports onto backup generators.

The Nuclear Regulatory Commission said that two nuclear reactors at Florida Power & Light’s Turkey Point station south of Miami automatically shut down, although it would not say why. Two coal plants at Turkey Point also shut down, possibly due to a chain reaction.

According to a news story on the incident, “The loss of power from a major station such as Turkey Point can trigger a cascade effect, causing other units to automatically shut when their circuit breakers sense a sudden drop or surge in power on the grid.”