A rare natural spectacle has captured the attention of both locals and tourists in Ha Long, Quang Ninh, after a stretch of seawater near Hon Gai turned reddish by day and glowed bright blue under motion at night - an event that's now gone viral on social media.

The seawater at Hon Gai turned red and glowed blue under light touch. Source: Nguyen Duc Manh/Light Music

A video shared online shows the unusual sight: a reddish patch of water near the beach lighting up with vibrant blue sparks when stirred by waves or human interaction. It quickly racked up hundreds of thousands of interactions.

Nguyen Duc Manh, a 23-year-old from Hanoi, was among the first to witness and document the moment while visiting Ha Long with friends.

Manh recounted that the group had gone to Hon Gai beach near Cot 8 gas station on the evening of May 17 to relax and play music. That’s when they noticed the sea had turned an unusual reddish color.

“I’d seen videos about red tides and bioluminescent water before but never imagined encountering it in Vietnam,” Manh said. “We looked it up and guessed it might be a red algae bloom, which isn’t harmful. When we touched the water, it lit up blue immediately.”

Their footage was taken between midnight and 2 a.m. on May 18.

“My friend who grew up in Ha Long said he’d never seen anything like it,” Manh added. “It was incredible - and thankfully, none of us experienced any itching or discomfort after touching the water.”

Tourists touched the glowing red water without experiencing any health issues. Source: Nguyen Duc Manh/Light Music

According to Assoc. Prof. Dr. Nguyen Duy Thinh, former lecturer at the Institute of Biotechnology and Food Technology, Hanoi University of Science and Technology, the reddish coloration was likely caused by a marine algae bloom - commonly known as a red tide.

“When algae reproduce rapidly in large numbers, they can turn seawater red, pink, purple, or even blue. This typically happens near bays, river mouths, and ports,” he explained. “Not all algae are toxic, but dense blooms can deplete oxygen in the water, threatening marine life. When the algae die, they may also cause foul odors.”

This phenomenon isn’t entirely new in Vietnam. In March 2024, a similar red tide was reported in Thien Cam and Cam Nhuong, Ha Tinh province.

Certain algae species also possess natural bioluminescence. When disturbed, they emit a bright blue glow - just like what was seen in the Hon Gai beach video. Additionally, some bioluminescent crustaceans like tiny shrimp or copepods may contribute to this luminous effect.

Glowing beaches caused by algae have previously been seen around the world, including in San Diego (USA), Vaadhoo and Mudhdhoo Islands (Maldives), and Xiangbi'ao Beach in China. However, red tides and glowing waves are unpredictable phenomena - no one knows exactly when they’ll begin or how long they’ll last.

Linh Trang