Bui Trong Anh, a staff member of the Essence Grand tourist cruise boat recorded a clip of the dolphin and shared it on his Facebook account, which was then spread widely on many social websites.

Speaking to the Lao Dong Newspaper, Anh said that it was 7am and the dolphin appeared near the Luon Cave. “It swam freely around the area for about 10 minutes before leaving," he said.

The man said that he has worked on the bay for over 13 years but this was the first time he has seen a dolphin here.

"There were some tourist boats operating in the area but the dolphin did not seem afraid," Anh added. "It looked so cute and I was very lucky to see it."

Luon Cave was among some tourist spots in the UNESCO World Heritage Site Ha Long Bay.

In January, a resident of the neighbouring Co To Island, while fishing for squid, recorded footage of a group of around 30 dolphins surfacing in the sea approximately 2-3 km off the coast of the island.

According to him, the encounter is not uncommon, but usually, only a few dolphins appear. The man said this is the largest group he has ever seen.

Last December, during their patrol, local border guard soldiers also saw a pod of whales appearing southeast of the island.

According to environmental experts, the reappearing of dolphins and whales near the shore after many years of absence has shown that the local marine environment has improved.