This Ghostly Fish Has Never Been Seen Alive Before. Until Now.

Scientists have uncovered another secret of the deep.

A fish that has never been seen alive before has been caught on camera swimming in the depths of the western Pacific Ocean's Mariana Trench.

The 4-inch-long ghostly looking creature, believed to belong to the Aphyonidae family, was glimpsed exploring a ridge 1.5 miles below the water's surface.

Okeanos Explorer, a National Oceanic and Atmospheric Administration ship, filmed the almost-transparent fish via one of its remotely operated underwater vehicles. Researchers posted the footage to YouTube on Friday.

"This is just remarkable," Dr. Bruce Mundy, fisheries biologist with the NOAA National Marine Fisheries Service, said in the now viral video.

"I am sure that this is an Aphyonid and I am sure that this is the first time a fish of this family has ever been seen alive," he added. "This is really an unusual sighting."

Mundy said fish belonging to the Aphyonidae family, which are in the same Ophidiiformes order as cusk eels, had previously only been seen dead -- after being brought up to the surface by deep-sea trawlers or dredgers.

"Some of us working with fish have wish lists, you know, a sort of bucket list of what we might want to see," Mundy added in the clip. "And a fish in this family was probably first on those lists for a lot of us."

The Okeanos Explorer team is surveying the deep-water areas of the Marianas Trench Marine National Monument and the Commonwealth of the Northern Mariana Islands until July 10. 

The scientists have been investigating fish habitats, mud volcanoes, ridges, deep-sea coral and sponge communities in a bid to further understand the extreme life present in the trench.