About 95 percent of Earth's oceans are unseen by human eyes, teeming with mystery that tends to deepen with the sea itself. Aside from sonar mapping, up to 99 percent of the sea floor is still unexplored, leaving us to imagine what might be down there.

That's finally changing, however, as scientists develop hardier probes that can go deeper — and record higher-definition video — than ever before. And thanks to one high-tech rover exploring insanely deep waters in the North Pacific, we now have HD video of a weird, "ghostlike" octopus that's apparently new to science.

On Feb. 27, a U.S. rover named Deep Discoverer ("D2" for short), was surveying the seabed in a remote area northwest of Hawaii. At a depth of 4,290 meters — more than 14,000 feet, or 2.6 miles, below the surface — its LED lights and HD cameras suddenly found themselves looking at this:

"This ghostlike octopod is almost certainly an undescribed species, and may not belong to any described genus," writes Michael Vecchione, a zoologist with the U.S. National Oceanic and Atmospheric Administration (NOAA), in a blog post about the discovery. "The appearance of this animal was unlike any published records."

Not only is it likely an unknown species, he adds, but it's also the deepest finless octopus ever seen. Octopuses come in two distinct groups — cirrate and incirrate — and deep-sea cirrate species (like dumbo octopuses) have side fins as well as fingerlike "cirri" on their suckers. Incirrate species lack both, and while they live at a variety of depths, many inhabit shallow waters and are thus more familiar.

D2's discovery belongs to the latter group, and immediately becomes the deepest-dwelling incirrate octopus ever documented. (Cirrate octopods have been reported as deep as 5,000 meters, but the deepest-known incirrate sightings — until now — were all shallower than 4,000 meters.) Yet despite having no fins or cirri, the newfound octopus still has a few weird features that help it stand out.

ghost octopusD2 found the ghost octopus while surveying near Hawaii's Necker Island, aka Mokumanamana. (Photo: NOAA)

Ghost and the machine

It "did not seem very muscular," according to Vecchione, and its mild muscle tone gives it a baggy, almost nebulous appearance. It also has no chromatophores, pigment cells that are typical of cephalopods, so its body is basically colorless. "This resulted in a ghostlike appearance," Vecchione writes, "leading to a comment on social media that it should be called Casper, like the friendly cartoon ghost."

Chromatophores are probably useless in such a low-light environment, anyway, he explains to Christine Dell'Amore of National Geographic, although the octopus's eyes still seem functional despite the dark — maybe to help it hunt bioluminescent prey.

"When the sub got up close to it, it started climbing away," he says, "either reacting to lights of the sub or vibrations of the water."

After seeing the octopus, Vecchione says he contacted two colleagues who agreed it's "something unusual" and that it sets a new depth record for incirrate octopods. "We are now considering combining this observation with some other very deep incirrate observations," he writes, "into a manuscript for publication in the scientific literature."

Finding an animal like the ghost octopus so deep below the surface demonstrates how far humans have come as ocean explorers, but as Vecchione points out to Dell'Amore, it also highlights how much we still have to learn.

We "don't know much about what lives in the deep sea," he says. "Because we have some opportunities to explore, we're finding these unexpected animals."

Okeanos Explorer shipThe Okeanos Explorer, seen here docked in Hawaii, is the base of operations for NOAA's D2 ROV. (Photo: NOAA)

In-depth research

To see more of this deep-sea frontier, you can follow D2's adventures online, as well as other aspects of NOAA's Okeanos Explorer mission. (The Okeanos Explorer is a converted U.S. Navy ship now dedicated to marine science; it's the platform from which D2 and its sister vehicle, Seirios, are operated.) There are live video feeds, mission logs, daily updates and a mobile app to let you tag along via smartphone.

Even if it's unusual to come across unknown octopuses, exploring the deep ocean often turns up some kind of otherworldly oddity — like this sea cucumber, spotted near Pioneer Bank in the Northwestern Hawaiian Islands on March 4:

sea cucumber swimmingThis transparent sea cucumber was seen swimming about 1,500 meters deep on March 4, 2016. (Photo: NOAA)

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.