The island of Sri Lanka lies twenty miles off the southeastern tip of India. Slightly smaller than Ireland, the country sits at the center of the Indian Ocean, just northeast of the Maldives island chain. Sri Lanka is divided into two geographic zones. The southwest of the island is Sri Lanka's core. It consists of the inland Central Highlands, flanked to the west by Colombo's seaside lowlands. This area receives most of the island's rainfall and is the country's agricultural heartland, producing rice for food consumption and its cash crops: coconut, tea and rubber. It also contains the island's gem and graphite deposits. Colombo has a deep natural harbor, which receives substantial traffic from nearby shipping lanes. The northern part of the island is divided into the inland Vanni region and the Jaffna Peninsula on Palk Bay. The north is dry and agriculture is only possible because of large, ancient reservoirs. Fishing is the primary economic activity. The north is connected to India by a chain of islands and sandbars known as Adam's Bridge, which has historically eased the movement of goods and people to and from the mainland. Sri Lanka's main geographic challenge is to protect its core around Colombo, while promoting integration with the north. The most recent challenge to Colombo's dominance was from the Tamil Tigers insurgent group, who used the Vanni region and Jaffna peninsula regions as bases to mount terrorist attacks. Uniting the north with the core will allow Colombo to take advantage of low-end manufacturing leaving China, using its underdeveloped north as a source of cheap labor.