Sri Lanka lies in the center of the Indian Ocean and thus has climatic and cultural links with three continents. Monsoon winds, driving against Sri Lanka's peaks, support lush vegetation on the southern half of the island, but the northern half is a dry zone. The winds affect human culture as well, having brought wave after wave of immigrants and merchants following the southerly trade routes. Outsiders found a wide range of ecological niches on the coast, on the plains, or in the mountains, and they built a remarkably variegated civilization. Merchants long have sought Sri Lanka as the source of pearls, jewels, spices, and tea. Visitors for centuries have marvelled at the beauty and great diversity of the island.