ruled out, as was landing troops by PBY Catalina flying boat on Lake Møsvatn, from the target, the latter due to the steepness of the surrounding mountain slopes and the inability of flying boats to land on the ice. It was decided that a raid by glider-borne airborne troops would have the greatest chance of success. The area around the plant would be difficult to land on, but a possible landing site for gliders was located near the Møsvatn dam, although it would require considerable skill on the part of the glider pilots in order to land safely. Preparations Operation Grouse In March 1942 the British Special Operations Executive (SOE) had recruited Einar Skinnarland, a Norwegian engineer who worked at the Møsvatn dam. Skinnarland had successfully sailed to Britain and was parachuted back to Telemark after ten days of intensive training. Having several contacts within Vemork he was able to roughly identify the disposition of German troops and other defences. Additionally SOE decided to send an advance party of Norwegian agents into Telemark, and began intensively training a four-man team over the summer. The party, code-named Operation Grouse, was led by Jens-Anton Poulsson and also included Knut Haugland, Claus Helberg and Arne Kjelstrup. The Norwegians, all people local to the area with exceptional outdoors skills, underwent extra outdoor training in Scotland as well as learning the skills necessary to operate in occupied territory; including sabotage, radio transmitting and "irregular warfare". The Grouse team were ready to be inserted by October. Several flights were made but aborted due to bad weather, before the team finally dropped on 18 October. The team landed at Fjarifet on the Hardangervidda (a large wilderness avoided by the German forces) and spent the next 15 days trekking towards Møsvatn, where they made contact with Skinnarland's brother, Torstein. Once they had established contact with London the party began making preparations for the arrival of the British commandos. A suitable glider landing site was chosen south west of Møsvatn dam and the team reconnoitred the area to help Combined Operations decide the best way to attack the plant. German defences The geography of the area around the village and the plant meant that attacking the plant and destroying the existing stocks of heavy water would be extremely difficult. Both were situated in a deep valley which possessed thickly–forested sides that rose almost vertically from a narrow river–bed, and