The Northwest Passage is a famed sea route from the Atlantic Ocean to the Pacific Ocean through a group of sparsely populated Canadian islands known as the Arctic Archipelago. European explorers first began to search for the Northwest Passage in the fifteenth century, but treacherous conditions and sea ice cover made the route impassible, foiling many expeditions. Norwegian explorer Roald Amundsen became the first to successfully navigate the Northwest Passage in 1906. Climate change has caused Arctic ice cover to thin in recent years, opening the passage to marine shipping. In summer 2007, the route was entirely ice-free for the first time in recorded history.