Amundsen's creditors as collateral for his mounting debt. Although they were unable to reach the North Pole, the scientific results of the expedition, mainly the work of Sverdrup, have proven to be of considerable value. Much of the carefully collected scientific data was lost during the ill-fated journey of Peter Tessem and Paul Knutsen, two crew members sent on a mission by Amundsen. The scientific materials were later retrieved by Russian scientist Nikolay Urvantsev from where they had been abandoned on the shores of the Kara Sea. Aerial Expeditions to the North Pole The 1923 attempt to fly over the Pole failed. Amundsen and Oskar Omdal, of the Royal Norwegian Navy, tried to fly from Wainwright, Alaska, to Spitsbergen across the North Pole. When their aircraft was damaged, they abandoned the journey. To raise additional funds, Amundsen traveled around the United States in 1924 on a lecture tour. In 1925, accompanied by Lincoln Ellsworth, pilot Hjalmar Riiser-Larsen, flight mechanic Karl Feucht and two other team members, Amundsen took two Dornier Do J flying boats, the N-24 and N-25, to 87° 44′ north. It was the northernmost latitude reached by plane up to that time. The aircraft landed a few miles apart without radio contact, yet the crews managed to reunite. The N-24 was damaged. Amundsen and his crew worked for more than three weeks to clean up an airstrip to take off from ice. They shovelled 600 tons of ice while consuming only of daily food rations. In the end, the six crew members were packed into the N-25. In a remarkable feat, Riiser-Larsen took off, and they barely became airborne over the cracking ice. They returned triumphant when everyone thought they had been lost forever. In 1926, Amundsen and 15 other men (including Ellsworth, Riiser-Larsen, Oscar Wisting, and the Italian air crew led by aeronautical engineer Umberto Nobile) made the first crossing of the Arctic in the airship Norge, designed by Nobile. They left Spitsbergen on 11 May 1926, flew over the North Pole on 12 May, and landed in Alaska the following day. Controversy over Polar Priority The three previous claims to have arrived at the North Pole: Frederick Cook in 1908; Robert Peary in 1909; and Richard E. Byrd in 1926 (just a few days before the Norge) are disputed by some, as being either of dubious accuracy or outrightly fraudulent. If these other claims are false,