Every once in a while, the ocean turns the color of blood and scores of dead fish rise to the surface. The phenomenon might look like a biblical plague, but the source is far more mundane. It's just algae.

Red tides occur when there’s a sudden population boom among specific kinds of algae, which in enormous quantities become visible to the naked eye. They occur all over the world. In the Gulf of Mexico, the culprit behind red tides washing onto coastlines from Texas to Florida is usually a type of microscopic algae called Karenia brevis. It produces toxic chemicals that can cause symptoms ranging from sneezing and eye irritation to disorientation, vomiting, and breathing difficulties. It's often fatal for fish, shellfish, turtles, and other wildlife.

The water appears red because of the particular depth at which the algae live. Light waves don’t penetrate seawater evenly, and certain wavelengths travel farther than others. The algae that cause red tides grow at depths that absorb green and blue frequencies of light and reflect red ones.

Not all algal blooms are red; some are blue, green, brown, or even purple. Nor do all algae harm humans or animals. Why and how certain species of algae multiply like crazy and wipe out entire swaths of marine life is still a scientific mystery.

The worst red tide on record occurred in 1946, when a mass of algae stretching for 150 miles along the Florida coastline killed more than 50 million fish, along with hundreds of dolphins and sea turtles. Tourists shied away from the beaches as the bodies of dead sea creatures washed ashore. Smaller incidents are more common, but just as costly. In the past decade alone, fishing and tourism industries in the United States have had an estimated $1 billion in losses due to red tides—and the cost is expected to rise.

Editor's note: This story, which originally ran in 2015, was updated in August 2018.