An ancient map of the North Atlantic - which features sea snakes and other dreadful monsters - may have boasted surprisingly advanced information.
By Julianna Kettlewell
BBC News Online science staff
The Carta Marina - published in 1539 - depicts elaborate sea swirls which, say researchers, closely match a giant ocean front shown in satellite images.
If correct, it means the Swedish cartographer Olaus Magnus may have been the first to map such an ocean feature.
Details of the assessment are published in the US marine journal Oceanography.
Boiling with monsters
The ornate Carta Marina is seemingly crude - and fanciful - by today's standards.
Scandinavia is alive with a rich panoply of beasts, doing all sorts of interesting things: wolves urinate against trees and stags rear wildly, while the sea off Scotland boils with dragons and monsters - many of whom are busily eating passing ships.
Olaus Magnus, an exiled Swedish priest living in Italy, was known to dislike blank canvas and covered every available space with ink. But Professor Tom Rossby, from the University of Rhode Island, US, believes not every elaborate quill stroke was artistic licence.
Most of the northeast Atlantic is drawn using more-or-less straight lines. However, things change off the east coast of Iceland, where the lines suddenly morph into a large group of whorls.
"Their location, size and spacing seem too deliberate to be purely artistic expression," Tom Rossby told BBC News Online. "Nowhere else on the chart do these whorls appear in such a systematic fashion."
When oceanographers from the UK's Plymouth Marine Laboratory and the University of Rhode Island compared these whorls with thermal images from an Earth observation satellite, they saw something surprising.
A thermal satellite image of the Iceland-Faroes Front (in blue)
Magnus's whorls corresponded almost perfectly with the Iceland-Faroes Front - where the Gulf Stream meets cold waters coming down from the Arctic.
"They are the earliest known description of large-scale eddies in the ocean - these are huge bodies of water, 100km in diameter, that turn slowly," Professor Rossby said.
"It seems the lines were deliberately drawn to aid navigation."
How did he do it?
Olaus Magnus was a keen traveller around northern Europe and spent time at sea with mariners operating in the northeast Atlantic. It seems likely that he learnt about the eddies from his conversations with them.
Any seaman familiar with the area would have been aware of the ocean eddies caused by the Iceland-Faroes Front.
The front creates contrasts in the water, which affects navigation. They would have noticed changes in water temperature and colour, wind direction, waves and swell.
"Magnus had been at sea with mariners so they may well have told him about these currents," Professor Rossby said.
"It is pretty obvious that he was a stickler for detail and it is my impression that these descriptions took root in his mind. People who are intrinsically curious - which he certainly was - will follow these things up.
"The effects of the front might have been common knowledge, which had just never been illustrated before."
One obstacle in the way of this theory is that Magnus himself never said the swirls were anything other than artistic decoration.
In fact, he never mentioned them at all. This seems strange considering he published a book, History Of The Nordic Peoples, in 1555, in which he refers to the map's tiniest detail, right down to the miniature ice skaters on a Scandinavian lake.
"That is certainly very strange," Professor Rossby conceded. "Maybe they are just artistic, but I don't think so. They are just too similar to the Iceland-Faroes Front."
Carta Marina images courtesy of the James Ford Bell Library