Editor’s note: We’ve updated this story to reflect that the video in this post was taken in 2020.

SAN DIEGO — Residents have spotted neon blue waves in San Diego again this week as the popular bioluminescence phenomenon returns to our shores.

Glowing surf has been spotted near Scripps Pier, in Del Mar and elsewhere around San Diego County since Tuesday, according to the Scripps Institution of Oceanography.

The dazzling display of bioluminescence goes hand in hand with what scientists call a “red tide.” It refers to a significant build-up of tiny organisms called dinoflagelattes, specifically the species Lingulodinium polyedra, Scripps explains.

This common member of the Southern California plankton family has a reddish-brown color. When it accumulates significantly — sometimes called a “bloom” — the water can take on a red tint on sunny days. That’s because the phytoplankton are gathering near the surface, scientists say.

At night, things take a turn for the spectacular. The churning sea jostles the tiny organisms, which produces a chemical reaction that emits a neon blue glow.

Once residents catch sight of the phenomenon, the timing window can be a little uncertain. Scientists say previous events have lasted anywhere from one week to a month or more. Scripps has been keeping an eye on this latest bloom since at least March 2.

After seeing reports about the return of this dazzling display, photographer Brian McClean shared this time-lapse video from 2020, which provides a similar view to those seen around San Diego in recent days:

If you’re trying to take a look for yourself, it’s best to wait until later in the evening.

“Bioluminescent displays are viewed best from a dark beach at least two hours after sunset, though visibility is not guaranteed,” Scripps explained in the 2020 blog post.

It’s difficult to say how often red tides occur, but scientists are developing tools that can help forecast their occurrence, Scripps research biologist Michael Latz said. Generally speaking, they are increasing in frequency, but more research is needed to understand the unpredictable phenomenon.

Generally, red tides are not thought to be harmful to humans, but some people are sensitive to the odor or to breathing in the air around them. The Southern California Coastal Ocean Observing System monitors the shoreline for any signs of toxic phenomena.