 Arab geographers were aware that the idea of the Indian Ocean as a closed basin was mistaken by the 8th century, for example in the work of al-Khwārizmī, "Book of the Description of the Earth".They showed that the Indian Ocean might be linked to the World Oceans, with the eastern limit of the inhabited world beyond the Malay Peninsula being the Island of the Jewel in the Sea of Darkness.The Ptolemaic eastern shore became the Dragon's Tail peninsula.