Blue whales are the biggest animals ever known, but it's still quite a job to find one.
They weigh up to 180 tonnes, reach 30m in length and eat up to 3.6 tonnes a day of what scientists assume is krill.
The behemoths are now the most high-profile target of Australian and New Zealand scientists aboard a New Zealand research vessel, Tangaroa, which set sail for Antarctica on Wednesday.
The reseachers have new equipment designed to listen underwater for the blue whales' `songs'.
Blue whales were decimated by commercial whaling in the 19th Century, and the Southern Ocean population of about 250,000 dropped to a few hundred before whaling was banned in 1964.
It may now have recovered to a few thousand.
Despite their size, the whales are fiendishly difficult to spot.
The International Whaling Commission says there have just been 200 sightings in 30 years.
Unlike humpback whales, which migrate past coasts, including through the Cook Strait, blue whales migrate through the open ocean and the only information about migration is from whaling data in the 1900s.
Some were tagged in the Southern Ocean a few years ago, but the tags didn't last long enough to record any migration.
They are also very difficult to count, says the Australian Antarctic Division's Mike Double.
"But we do have some data now to say they are recovering but they are recovering very slowly."
The expedition scientists are confident they won't be looking for a needle in a haystack.
"We have learned how to find these whales," Dr Double says.
That involves dropping sonobuoys in the water to listen for the whales' song and then triangulation can pinpoint their location.
Humpbacks are prolific singers but by comparison the blue whale song is very boring, Dr Double says.
"It's very low-frequency monotone, occasionally it does the odd squeak ... but to call it a song is very generous," Dr Double said.
The song travels great distances underwater.
"We don't have to go far from Wellington before we start hearing whales that are singing in the Antarctic. It's that powerful."
Once the whales are found - possibly aggregating in hotspots - the scientists hope to find out why they are there and what they are eating.
If the whales swim close enough to the Tangaroa, samples may be taken and GPS tags attached.
However, before the ship starts tracking the blue whales, it will head to the Balleny Islands - a string of islands about 200km from Antarctica, which are made up of just ice and rock - a hot spot for feeding humpback whales.
"We don't even know what they eat down there," says voyage leader, Richard O'Driscoll, from New Zealand's National Institute of Water and Atmospheric Research, known as NIWA.
It could be krill, or a small cod called the Antarctic silverfish, which is regarded as the only true pelagic fish in Antarctic waters and an important food species.
That fish is also something of a mystery. An echo sounder, to be moored in Antarctica's Terra Nova Bay, will hopefully catch evidence of their life cycle.
Their eggs and larvae are in the water, but it's not known if the fish spawn under the ice or if the eggs and larvae drift down from further north.
The echo sounder will hopefully catch the fish as they come into the area.
The other fish under investigation is the toothfish and its prey, icefish and grenadiers, in the Ross Sea.
Twenty-one scientists from the NIWA, Antarctica NZ and the Australian Antarctic Division are on board Tangaroa.
The expedition is costing $A3.7 million and the scientists say it will be well worth it for the information gathered.
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