Brian Jeffrey
An extraordinary natural occurrence has fascinated both residents and visitors in Ha Long, Quang Ninh, where a section of seawater near Hon Gai turned red during the day and emitted a bright blue glow when disturbed at night—an event that has since gone viral on social media platforms.
The water in Hon Gai exhibited a reddish hue and glowed blue when touched. Source: Nguyen Duc Manh/Light Music
A widely circulated video captures this remarkable phenomenon: a reddish section of the water illuminating with electric blue sparks when agitated by waves or human contact. It garnered a rapid response, amassing hundreds of thousands of views.
Nguyen Duc Manh, a 23-year-old from Hanoi, was among the first to document the event while visiting Ha Long with a group of friends.
Manh shared that they had gone to Hon Gai beach, close to the Cot 8 gas station, on the evening of May 17 to unwind and enjoy music when they noticed the unusual red tint of the sea.
“I had seen videos about red tides and bioluminescent water previously, but I never expected to experience it in Vietnam,” Manh remarked. “We researched and speculated it was probably a non-harmful red algae bloom. When we touched the water, it illuminated bright blue right away.”
The footage was recorded between midnight and 2 a.m. on May 18.
“A friend from Ha Long mentioned he had never seen anything like this,” Manh added. “It was astonishing—fortunately, none of us felt any irritation or discomfort after touching the water.”
Visitors interacted with the glowing red water without any health concerns. Source: Nguyen Duc Manh/Light Music
As per Assoc. Prof. Dr. Nguyen Duy Thinh, a former lecturer at the Institute of Biotechnology and Food Technology at Hanoi University of Science and Technology, the reddish coloring is likely due to a marine algae bloom, commonly referred to as a red tide.
“When algae proliferate rapidly, they can give the water reddish, pinkish, purple, or even bluish hues. This typically occurs near bays, river mouths, and ports,” he explained. “Not all algae are harmful; however, dense blooms can deplete oxygen levels in the water, posing risks to marine life. When these algae die off, they may also result in unpleasant odors.”
This phenomenon isn’t entirely new to Vietnam. A similar red tide was noted in March 2024 in Thien Cam and Cam Nhuong, in Ha Tinh province.
Some species of algae also display natural bioluminescent properties. When disturbed, they emit a brilliant blue glow—like the phenomenon recorded on Hon Gai beach. Additionally, certain bioluminescent creatures, such as small shrimp and copepods, may also contribute to the glowing effect.
Bioluminescent beaches caused by algae have been seen globally, including locations like San Diego (USA), Vaadhoo and Mudhdhoo Islands (Maldives), and Xiangbi’ao Beach in China. However, both red tides and glowing waves are unpredictable events—no one can precisely determine when they will occur or how long they will last.
