Sumatra is an elongated landmass spanning a diagonal northwest–southeast axis. The Indian Ocean borders the west, northwest, and southwest coasts of Sumatra, with the island chain of Simeulue, Nias, Mentawai, and Enggano off the western coast. In the northeast, the narrow Strait of Malacca separates the island from the Malay Peninsula, which is an extension of the Eurasian continent. In the southeast, the narrow Sunda Strait, containing the Krakatoa Archipelago, separates Sumatra from Java. The northern tip of Sumatra borders the Andaman Islands, while off the southeastern coast lie the islands of Bangka and Belitung, Karimata Strait and the Java Sea. The Bukit Barisan mountains, which contain several active volcanoes, form the backbone of the island, while the northeastern area contains large plains and lowlands with swamps, mangrove forest and complex river systems. The equator crosses the island at its centre in West Sumatra and Riau provinces. The climate of the island is tropical, hot, and humid. Lush tropical rain forest once dominated the landscape.