Results of the first ever global marine life census have been unveiled, revealing a startling overview after a decade-long trawl through the murky depths.
The Census of Marine Life estimates there are more than one million species in the oceans, with at least three-quarters of them yet to be discovered.
The US$650 million (A$667 million) international study discovered more than 6000 potentially new species, and found some species considered rare were actually common.
The study authors says it offers "an unprecedented picture of the diversity, distribution and abundance of all kinds of marine life in Planet Ocean - from microbes to whales, from the icy poles to the warm tropics, from the tidal near shores to the deepest dark depths."
The census establishes a baseline against which 21st-century changes can be monitored.
New species were discovered, marine highways and rest stops mapped and changes in species abundance were documented.
The research involved more than 2700 scientists, 670 institutions, more than 540 expeditions and around 9000 days at sea. Nearly 30 million observations of 120,000 species were made.
The census was formally launched in London, with more than 300 figures involved gathering to share the results and consider their implications.
"The census has far exceeded any dream that I had. We felt like the people who created the first dictionary and encyclopaedia 250 years ago," says Jesse Ausubel, a scientist who co-founded the study.
"The most surprising thing was beauty. Our eyes pumped out of our head in front of this beauty."
The survey set out to find out what used to live in the oceans, what lives there now and what might live there in the future.
It found 16,764 species of fish had so far been described, but an estimated 5000 more were yet to be discovered.
Scientists found some species thought extinct 50 million years ago, while other finds were less encouraging.
Around 40% of plankton, at the bottom of the ocean food chain, has disappeared in the last 30 years, which was put down to a rise in ocean temperatures.
Sharks have disappeared from 99% of some areas.
Australian Ian Poiner, chair of the census steering committee, says the researchers "systematically defined for the first time both the known and the vast unknown, unexplored ocean".
"All surface life depends on life inside and beneath the oceans. Sea life provides half of our oxygen and a lot of our food and regulates climate. We are all citizens of the sea," he says.
"While much remains unknown, including at least 750,000 undiscovered species and their roles, we are better acquainted now with our fellow travellers and their vast habitat on this globe."
The census documented a changing marine world, richer in diversity, more connected through distribution and movements, more impacted by humans and less explored than was expected.
The researchers used sound, satellites and electronics to track migratory routes.
They got down to 10 kilometres below the sea in the western Pacific Ocean's Mariana Trench.
They affirmed that by weight up to 90% of marine life is microbial.
Scientific steering committee vice-chair Myriam Sibuet of France, says: "The census enlarged the known world. Life astonished us everywhere we looked.
"In the deep sea we found luxuriant communities despite extreme conditions. The discoveries of new species and habitats both advanced science and inspired artists with their extraordinary beauty."
Much of the marine world remains to be explored, so vast are the seas.
The census shows where explorers have not yet looked, with little data for large areas of the oceans and no data at all for more than 20% of the oceans' volume.