A Norwegian team has embarked on an expedition to find the submerged wreck of a plane which carried Norway's great polar explorer Roald Amundsen.
Amundsen was aboard a Latham 47 flying boat when the aircraft disappeared over the sea on its way to the Arctic island of Spitsbergen in 1928.
Two ships have set sail from the Norwegian city of Tromso to begin the two-week expedition.
Underwater robotic vehicles will be used to scan for the plane using sonar.
Between 1910 and 1912, Amundsen led the first expedition to reach the South Pole, reaching the target some five weeks before his British rival Robert Scott.
On 18 June 1928, Amundsen joined a rescue operation to save another competitor, Umberto Nobile.
The Italian aviator had crashed his airship Italia on a return voyage from the North Pole. Nobile and his surviving crew members found themselves drifting helplessly on pack ice.
Amundsen boarded a Latham 47 flying boat along with a team of French Air Force pilots to try to reach them.
According to experts involved in the 2009 expedition, the Latham 47 should have been about 19 nautical miles south of Bear Island when the plane's last radio message was picked up at 1845 on 18 June.
The 2009 search is led by New Zealander Rob McCallum, a veteran expedition leader and project co-ordinator.
Mr McCallum told BBC News: "We are using every bit of intelligence we can gather in order to define the search area and we are using the best assets available.
"We are using equipment that can get down to 20cm resolution (on the sea bed). So we can detect very small items indeed."
"If we can't detect anything within that search area, then the mystery will probably remain forever, because Amundsen could be anywhere within the Barents Sea."
Mr McCallum said all that might remain of the plane after so many years are its engines, because the rest of the aircraft was made of perishable materials such as plywood.
Another team member is Nicolay Jacobsen, a great-nephew of Amundsen. Mr Jacobsen said: "If we were to find something, it would be really amazing... I feel really excited and the rest of the family are too."
He added: "If we can find some leads to strengthen some theories, or can help in some way to find out what actually happened, then it will be even more interesting."
Referring to Amundsen, he told BBC News: "[Norway was] a young country, fighting for its place on the map. He gave people something to be proud of."
Two ships are working together on the search: the Royal Norwegian Navy vessel KNM Tyr and its larger supply vessel, the Norwegian Coast Guard ship KV Harstadt.
Together, are scouring some 117 sq km (45 sq miles) of sea floor for the downed plane. State-of-the-art technology is crucial to the search.
The KNM Tyr set sail at 0830 BST on Monday, while the KV Harstadt left Tromso at 1800 BST.
The expedition is using an autonomous underwater vehicle (AUV) called Hugin 1000, capable of making high-resolution maps of the sea bed.
The sonar-equipped AUV can function independently for about 12 hours. After this, it needs to be brought aboard the Tyr for the data to be downloaded.
The Tyr also deploys a tethered underwater robot called Scorpion 21. This remotely operated vehicle (ROV) is equipped with an HDTV camera and receives power from the ship.
This means it can be used in the water for as long as necessary.
A camera crew from German production company Context TV are filming the search efforts for a documentary.
"The story of Amundsen and Nobile is not very well known... lots of people know about the South Pole, but not very many people know what happened after that," said Daniel Petry, the chief executive of Context TV, who is also joining the expedition.
"The secret to the success of Amundsen's expeditions was preparation. [The rescue mission for Nobile] was probably the first one that had not been well-prepared."
Mr Petry told BBC News that nobody knew exactly what had happened on that evening in June. But if, for example, the plane had flown into foggy conditions, the crew might have been forced to land on water.
He said the Latham 47 had not been built to cope with the rough conditions which can be encountered in the Barents Sea and could have been overcome by waves.
Several items belonging and purporting to belong to the plane have been found over the years. In August 1928, during the original rescue operation for Amundsen and the French crew, the left wing pontoon of the Latham 47 flying boat was recovered from the sea.
A piece of the fuel tank was found a couple of months later near Trondheim in Norway.
A trawler might have recovered one of the Latham's engines in 1933. Unfortunately, it disappeared into the sea again. Then, in 1964, a double sheet of plywood which might have belonged to the aircraft was found near Svalbard.