Beachgoers in California were treated to a rare spectacle this week: glowing tides.
The unique sight is called a red tide, but, at least at night, it results in an other-worldly blue light in the water. Numerous people spotted and photographed the light show off the coast of San Diego this week.
The phenomenon is caused by high concentrations of tiny aquatic organisms called dinoflagellates, according to the University of California San Diego’s Scripps Institute of Oceanography. By day, these organisms turn the water red or brown. But at night, according to the Scripps Institute, the organisms create bioluminescence in waves that break on the shore.
The effect appears to have mostly petered out in California, Scripps says, and there’s no way to predict when or where it will return. They are, however, fairly rare — one was last observed in California in 2013, the Los Angeles Times reports.
Couldn’t see the spectacle for yourself? Check it out in the video above.