Before aircraft made travelling the world much easier, the constant evolution of naval exploration had allowed people to explore further than ever before. As well as the military importance placed on a country's navy, merchant ships also made trade and commerce much more prevalent, particularly for exotic goods. The Vikings were famous for their advanced wooden boats and longships, while the British Empire and its Royal Navy were the most formidable naval force for centuries. Armoured battleships were introduced in the late 19th century, while submarines were widely used during the 20th century. Many different types of ships, from aircraft carriers to cruisers, still play a vital role in modern day militaries.