[Photo by Jim Hellemn/China Daily]

Last year, on a remote island in the South Pacific, Gruber and his friend were surprised to come across a large group of fish glowing under shallow water about one finger deep. Usually, these glowing species are deep in the ocean where one needs to use a submarine to reach. Thousands of the shimmering fish swarmed around one of his friends, who had a 3-D camera and a mirror.

"I think it was the first time the fish saw themselves in a mirror. So amazing for us to see such density of flash-like fish in such a shallow depth," says Gruber, who plans to return to the same place in August hoping to see them again.

The deepest Gruber has ever traveled is about 1,000 meters under the sea. The average depth of the ocean is about 4,000 meters. Gruber was with two other men in a submarine. They stayed there for about seven hours. They turned off all the lights to give themselves a better chance to find the glowing species.

"It felt like we were the first to travel into the area," he says.

He even swam with more than 100 reef and tiger sharks early one morning in the Bahamas when conducting research on fluorescent sharks. Actually, the sharks were very shy and they are misunderstood by humans as a dangerous fish.

"There are about 600 species of sharks. Most of them have no idea they have such a bad reputation," jokes Gruber, who was selected as National Geographic's emerging explorer in 2014 and dubbed one of the new generation of explorers.

"Unlike the older generation that collected marine animals and showed them off at their houses, we mainly use cameras, sharing photos with the public to raise awareness on ocean protection," he says.