What was the Northwest Passage and why were countries so interested in finding it?

The Northwest Passage was a theoretical sea route through North America from the Atlantic to the Pacific, which exists only by way of the Arctic Ocean in northern Canada. European colonial powers unsuccessfully sought a river or bay passage that would allow ships to sail from Europe to Asia without going south around South America. The closest in central North America is the Missouri River, which is not entirely navigable. The northern route through Baffin Bay is blocked by sea ice for most or all of the year, only becoming seasonally ice-free by the 21st century.