They are the stuff of children's nightmares, legends and myths and there is still very little known about the colossal squid - except that it holds a very unusual world record.
Photos of the colossal squid are extremely rare, this one was caught by fishermen in the Antarctic Sea in 2007Image: picture-alliance/dpa/NZPA/Minister of Fisheries
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With satellite imaging, GPS and internet available in even the remotest corners of the globe, it sometimes feels like there is little left to explore in this world. But drop just the length of a football field below the surface of the ocean and you'll be humbled by how little we still know about what's out - or rather down - there.
Take the colossal squid for example. It's one of those deep-sea dwellers bound to give small children nightmares. The alien-looking creature has tentacles equipped with sharp hooks, a massive beak and can grow as long as 10 meters and weigh as much as half a metric ton 500kg (1100lbs) - as far as we know. So few of these creatures have been captured or found (around 10) that much about the way they live - or how big they can actually get - is still unknown.
This is what the colossal squid would probably look like if scientists were ever able to get a good picture of the elusive creature. The shadow of the diver gives you a sense of how massive the animal is.Image: Citron/CC BY-SA 3.0
As one researcher put it aptly at a conference: "Trying to observe one in the ocean is like jumping out of a plane with a parachute somewhere above North America at night, armed with a flashlight, hoping to find a grizzly bear."
Part of the problem is also that the deep sea dwellers don't fare well once they get dragged to the surface and raising young ones in captivity hasn't been successful so far.
Aside from being massive, the biggest colossal squid ever found has set another record: It has the biggest eyes of any known animal. Almost 30 centimeters in diameter, they are bigger than a professional men's basketball.
Why they are so big and why they are much bigger than the eyes of its similarly-sized relative, the giant squid, is yet another mystery waiting to be solved.
Wonders of the High Seas
Unesco experts have come up with five spots far out in the oceans that they think should be added to the list of World Heritage Sites - even though most people will likely never see them. Here are their suggestions.
Deep down, far-off and simply amazing
Five spots in the high seas should be deemed World Heritage sites, Unesco experts recommend. They say their beauty is breathtaking, their biodiversity tremendous and they are important ecosystems for endangered wildlife.
Image: Kevin Raskoff/NOAA/Wikipedia
1. The Lost City Hydrothermal Field
This geo-biological feature lies in 800-meter deep water in the Atlantic Ocean, on a volcanic hotspot. UNESCO experts say it is "unlike any other ecosystem yet known on Earth". Hydrothermal vents towering out of the seabed like chimneys have been emitting gases, probably for more than 120,000 years. A 60-meter high monolith called "Poseidon", made of carbonate, is the dominant feature.
Image: Woods Hole Oceanographic Institute and Charles Fisher, Pennsylvania State University
2. The Costa Rica Thermal Dome
UNESCO calls this area, extending over up to 500-kilometers in the Eastern Tropical Pacific, an "oceanic oasis". The habitat attracts large fish, marine mammals and predators like sharks, tuna, dolphins and whales - most notably the blue whale (shown here). Critically endangered leatherback turtles often pass by on their way to other parts of the world.
Image: picture alliance/WILDLIFE
3. The White Shark Café
Yes, marine biologists really did come up with this name! Of course it is not a coffeehouse, but an open ocean region, halfway between the North American mainland and Hawaii. To the human eye, it might seem featureless and unremarkable. But obviously not to the white shark. Large numbers of white sharks congregate here to feed and mate. The currents seem to attract them.
Image: Pterantula (Terry Goss) via Wikimedia Commons
4. The Sargasso Sea
Columbus is said to have viewed this spot on his first voyage in 1492. The Sargasso Sea surrounds the islands of Bermuda. Its special feature is the floating Sargassum seaweed, which forms what marine biologists describe as a 'Golden Floating Rainforest'. It hosts many other living organisms, and is the only breeding location for European and American eels.
Image: picture-alliance/AP Photo/Courtesy Chris Burville
5. The Atlantis Bank
Steep cliffs, stacks, beaches and lagoons at a depth of 700 to 4000 meters, populated by large anemones, armchair-sized sponges and corals - that is the Atlantis Bank in the Indian Ocean. It was originally an island that sank to the bottom of the ocean millions of years ago. Atlantis? Amazing!
Image: The Natural Environment Research Council and IUCN/GEF Seamounts Project C/O Alex D Rogers
Outstanding universal value
All five places lie in international waters, outside of any national jurisdiction. That's why they are hard to protect. Deeming them World Heritage, though, would be a way, experts say.