Bromo Tengger Semeru National Park (; abbreviated as ) is a national park located in East Java, Indonesia, to the east of Malang and Lumajang, to the south of Pasuruan and Probolinggo, and to the southeast of Surabaya, the capital of East Java. It is the only conservation area in Indonesia that has a sand sea, the Tengger Sand Sea (), across which is the caldera of an ancient volcano (Tengger) from which four new volcanic cones have emerged. This unique feature covers a total area of 5,250 hectares at an altitude of about . The massif also contains the highest mountain in Java, Mount Semeru (), four lakes and 50 rivers. It is named after the Tengger tribe. The explosion of the volcano that created the caldera, happened ca. 45.000 years ago, in an event similar to the Krakatau eruption. The Tengger Sand Sea has been protected since 1919. The Bromo Tengger Semeru National Park was declared a national park in 1982. Geography Tengger massif The Tengger massif is a massif within the park. The area is an active volcanic complex surrounded by a plain of sand. Volcanism The volcanic complex of Tengger forms a condition where a new caldera of volcano forms inside a larger and more ancient caldera. There are five volcanoes inside the Tengger Caldera: Mount Bromo (2,329 m), Mount Batok (2,470 m), Mount Kursi (2,581 m), Mount Watangan (2,661 m), and Mount Widodaren (2,650 m). Mount Batok is the only peak that is no longer active, and is covered in casuarina () trees. Mount Widodaren, located beside Mount Batok, contains the cave Widodaren, which is considered sacred by local people. The five volcanoes within the caldera are surrounded by a vast area of sand called the Tengger Sand Sea, which in turn is surrounded by a steep crater wall of the larger Tengger Caldera with height differences of about . Other mountains around the Tengger caldera are: Mount Pananjakan (2,770 m), Mount Cemorolawang (2,227 m), Mount Lingker (2,278 m), Mount Pundak Lembu (2,635 m), Mount Jantur (2,705 m), Mount Ider-ider (2,527 m) and Mount Mungal (2,480 m). The peak of Mount Pananjakan is the most popular place to watch the entire volcanic complex of Tengger. Further south in the national park, there is another volcanic complex called the Semeru Group or Jambangan Group. This area contains the highest peak of Java, Mount Semeru (3,676 m).