Kublai Khan was the grandson of Genghis Khan, and ruled the Mongol Empire from 1260 to 1294. His huge realm included Mongolia, China, Tibet, Korea, Afghanistan, Iraq, Siberia and even parts of Eastern Europe such as Poland. He also attempted unsuccessful invasions of Japan and Vietnam. A notable fact about him is that he was the first world leader to introduce paper money as currency. Italian explorer Marco Polo wrote of visiting the city of Xanadu, in China, during Kublai Khan's reign. He claimed that the ruler "rides through the park with a leopard behind him on his horse's croup". One of Samuel Taylor Coleridge's most famous poems is called 'Kubla Khan'; he said he was inspired to write it after taking opium.