Beachgoers in California were treated to a rare spectacle last Saturday with eerie blue glowing tides. It may look otherworldly, but this striking display of light seen on some beaches in Southern California this week has a very earthly explanation.

The unique sight is called a red tide. The San Diego coast is hosting a red tide, which is a bloom involving a population explosion of millions of tiny single-celled organisms that can make the water appear red during daylight hours.

However at night, even tiny movements caused by fish or waves causes these little organisms to emit a flash of blue light.

The Scripps Institution of Oceanography sharde a photo on Wednesday showing a glowing blue light coming from the waves near a beach in San Diego. So by day, these organisms turn the water reddish-brown. But at night, the organisms create bioluminescence in waves that break on the shore. The gentle glow is reminiscent of views seen of auroras of the Northern Lights.

The National Ocean Service prefers to call these potentially toxic occurrences "harmful algal blooms." although Scripps says that this particular red tide in the California coastal area is not known to be toxic. However care should be taken as some people may be sensitive should they come into contact with it.