Indonesia is the largest archipelago in the world. It consists of five major islands and about 30 smaller groups. The total number of islands, according to the Indonesian Naval Hydro-Oceanographic office, is 17,508. The archipelago is at the crossroads of two oceans, the Pacific Ocean and the Indian Ocean, and bridges two continents, Asia and Australia. This strategic position has always influenced the cultural, social, political, and economic life of the country. 

The territory of the Republic of Indonesia stretches from 6.08' north latitude to 11.15' south latitude. The Indonesian sea area is four times greater than its land area, which is about 1.9 million sq km (including an exclusive economic zone) and constitutes about 81% of the total area of the country. 

The five main islands are: Sumatra, which is about 473,606 sq km in size; Java/Madura, the most densely populated islands, 132,107 sq km; two-thirds of the island of Kalimantan measuring 539,460 sq km; Sulawesi, 189,216 sq km; and Papua, 421,981 sq km, which is part of the world's second largest island, New Guinea. Indonesia's other islands are smaller in size. 

Located between these two shelves is the island group of Nusa Tenggara, Maluku and Sulawesi, where the sea depth reaches 15,000 feet. Coastal plains have been developed around the islands of Sumatra, Java, Kalimantan and Papua. 

The land area is generally covered by thick tropical rain forests, where fertile soils are continuously replenished by volcanic eruptions like those on the island of Java. 

The country is predominantly mountainous, with about 400 volcanoes, 100 of which are active. Mountains higher than 9000 feet are found on the islands of Sumatra (Mt. Leuser and Mt. Kerinci); Java (Mt Gede; Mt. Tangkubanperahu, Mt. Ciremai, Mt. Kawi, Mt. Kelud, Mt. Semeru and Mt.Raung); Sulawesi (Mt. Lompobatang and Mt. Rantekombala); Bali (Mt. Batur and Mt. Agung); Lombok (Mt. Rinjani) and Sumbawa (Mt. Tambora). The highest mountain is the perpetually snow-capped Mandala Top (15,300 feet) in the Jaya Wijaya mountain range of Papua. 

Many rivers flow throughout the country. They serve as useful transportation routes on certain islands, for example, the Musi, Batanghari, Indragiri and Kampar rivers in Sumatra; the Kapuas, Barito, Mahakam and Rejang rivers in Kalimantan; and the Memberamo and Digul rivers in Papua. On Java, rivers are important for irrigation purposes, including the Bengawan Solo, Citarum and Brantas rivers. 

A number of islands are dotted with scenic lakes, like the Toba. Danau Toba, or Lake Toba as we know it, is the largest lake in Southeast Asia. It was created by the eruption of a super volcano 75 thousand years ago. It is still surrounded by the crater edge of that volcano, and in the middle of the lake, volcanic activity created Samosir an island as big as Singapore. 

Other famous lakes are Maninjau and Singkarak lakes on Sumatra; the Tempe, Towuti, Sidenreng, Poso, Limboto, Tondano, and Matana lakes on Sulawesi; and the Paniai and Sentani lakes on Papua.