The second reason is technological.
Marine biologists find about 2,000 new species a year, a rate hardly changed since Darwin’s day.
Ocean Census is betting it can go faster.
“Cyber taxonomy”, for instance, involves feeding DNA sequences from animals into computers, which can quickly decide whether it is a new species.
The ability to describe new creatures, as well as simply cataloguing them, has also improved.
Fancy cameras on remote-operated vehicles, for instance, allow scientists to make laser scans of deep-sea creatures such as jellyfish without removing them from their habitat.
Just as the immense pressures of the deep sea are fatal for humans, taking such a jellyfish to the surface for examination reduces it to gooey slime.
Ocean Census is not the first attempt to conduct a systematic survey of life in the oceans.
The Census of Marine Life was a ten-year effort, begun in 2000, to seek out new species.
The Global Ocean Sampling Expedition, which ran from 2004 to 2006, aimed to catalogue microbial life in the sea by sampling waters from across the world.
(It was funded by Craig Venter, a biologist-cum-entrepreneur, and carried out on his personal yacht.)
Exactly what the new effort might turn up, of course, is impossible to predict.
But history suggests it will be fruitful.
Half a century ago scientists discovered hot vents on the sea bed that were home to organisms living happily in conditions that, until then, had been thought inimical to life.
These days, such vents are one plausible candidate for the origin of all life on Earth.
There are more practical benefits, too.
Many drugs, for example, come originally from biological compounds.
An ocean full of uncatalogued life will almost certainly prove a rich seam from which to mine more.
One type of marine snail, Conus magus, was recently discovered to produce a painkilling compound 1,000 times more potent than morphine.
To help make use of its data, Ocean Census plans to make it freely available to scientists and the public, who will be able to scour it for anything useful or surprising.
The point of exploration, after all, is that you never know what you might find.