An ethereal blue glow lights up the beach near Beigang village, Pingtan, on the evening of April 7. [Photo provided to chinadaily.com.cn]

An ethereal blue glow lit up the beach near Beigang village in Pingtan, Fujian province, on the evening of April 7, sparking debates in cafes and chat rooms across the city about what could have caused the strange phenomenon.

The ghostly light was first spotted by Tang Weiyao, a native of the northern Chinese city of Tianjin who is currently working at a café named Laocai in Beigang.

A keen amateur photographer, Tang snapped several photos of the bright azure sea. When his manager at Café Laocai, Liu Dan, posted the photos to the popular Chinese social media platform WeChat, it sparked a frenzy of speculation about the source of the intriguing illuminations.

Some Pingtan local netizens were thrilled and guessed the eerie blue glow might be "Blue Tears".

Blue Tears refers to rare phenomenon first witnessed in Pingtan in August 2013, which is caused by a myriad of ostracods, small crustaceans that are washed onto the island city's beaches by the rising tides. The flickering blue lights produced by these little creatures light up the night, creating making the sky look as if it is glowing blue.

However, local oceanologists dismissed this idea, and suggested the light was emitted by noctiluca scientillans, a kind of marine plankton that exhibits bioluminescence when disturbed: A phenomenon popularly known as milky sea or mareel.