The English word "junk" owes its origin to the Javanese "jong" that refers to a "big vessel" or "ship". Chinese junks appear as early as during the Han Dynasty (220 BC–200 AD) and were part of the imperial fleets. Junks were used extensively for trade and transportation. The military and mercantile power of Chinese rulers depended heavily on these junks. It is believed that the largest junks were built by Admiral Zheng He (1371–1433), a Chinese mariner, which were used for his Indian Ocean expeditions. According to Chinese sources, this fleet consisted of 30,000 men and more than 300 ships when it was at its height.