For centuries, European explorers, beginning with Christopher Columbus in 1492, sought a navigable passage as a possible trade route to Asia, but were blocked by North, Central, and South America, by ice, or by rough waters (e.g. Tierra del Fuego). An ice-bound northern route was discovered in 1850 by the Irish explorer Robert McClure whose expedition completed the passage by hauling sledges. Scotsman John Rae explored a more southerly area in 1854 through which Norwegian Roald Amundsen made the first complete passage entirely by ship in 1903–1906.