LURKING, often thousands of feet deep below the ocean surface, are creatures that sometimes don't look like they should be earthbound.

Humans are becoming well-acquainted with these alien-like creatures with the help of increasingly sophisticated underwater robots.

This creepy creature, known as the barreleye fish, lives in depths of up to 1km (3280ft) in the northern Pacific Ocean, where the light rarely touches.

The two bright green orbs that are visible through the see-through dome on its forehead are its eyes.

It is one of the weirdest-looking fish in the world, and was caught on camera by researchers at the Monterey Bay Aquarium Research Institute (MBARI) earlier this year.

With its transparent skull and glowing green eyes, the barreleye specialises in stealing prey from jellyfish.

Its tube-shaped eyes are like tiny binoculars, with tunnel vision that can focus on minute details.

The barreleye, also known as the Macropinna microstoma, is only a few centimetres in length.

The rarely seen bigfin squid and its 13ft tentacles was filmed by an underwater camera some 20,000ft below the oceans surface earlier this year.

They're known for their long tentacles that can grow up to 26ft long.

Only about a dozen sightings of them have been confirmed worldwide.

Researchers came across the terrifying creature after they lowered a camera to the bottom of the Tonga Trench in the South Pacific ocean.

"We always hope to see this type of animal," Alan Jamieson, a professor and deep-sea scientist at the University of Western Australia who collected the footage, told Live Science in an email in late September.

"[Bigfin squid] are not something you would actively go looking for, they are a species that relies on us coming across them by accident."

Most documented bigfin squid sightings are "serendipitous filming from oil and gas activities," Jamieson said.

The dinner plate jellyfish, also known as Solmissus sp., is one of the most dominant predators in the ocean's inky depths.

It has a fairly unfussy appetite.

"This denizen of the deep has an appetite for other gelatinous animals. Jellies, comb jellies, siphonophores, salps - they are all on the menu for Solmissus," researchers at MBARI said in an Instagram post.

"In fact, this jelly has one of the most diverse diets of all midwater animals - so far, we have seen Solmissus eating 21 different types of gelatinous prey."

A hungry dinner plate jellyfish swims with tentacles held forward to catch its prey by surprise.

"Forward-pointing tentacles also help the dinner plate jelly catch animals with long tentacles or skinny bodies, like raking up twigs in the lawn," MBARI researchers noted.

Perched atop coral or rocks, this bizarre-looking creature looks like it has been pulled out of Pans Labrinth.

It's swirling arms, which can sometimes look like tree roots or fern leaves, are used to snag food like zooplankton.

These branches then pass food towards its mouth, beneath the star's central disk, according to MBARI researchers.

These deep-sea creatures can live up to 35 years old, and weigh as much as 11 pounds.

With eight long and spindly legs, this creepy crawly, known as the giant sea spider, sticks to the deep seafloor.

While it lurks at depths of between 2,200m and 4,000m (7,200ft -13,100ft), these creatures can be found worldwide.

"Instead of spinning a delicate web of silk to trap prey, a giant sea spider uses an elongate, tube-like proboscis to slurp up its prey," according to researchers at MBARI.

It feasts on creatures such as sea anemones, hydroids, jellies, and other invertebrates.

The spider, whose official name is Colossendeis sp., can measure up to 20 inches (51cm) - much larger than its land-walking cousins.

By comparison, the Goliath birdeater - the largest spider on land - has a body length of roughly 13cm.