Mount Bromo in front and Mount Semeru far behind.
This national park is named after its two mountains, Mount Semeru (the highest in Java at 3,676m), Mount Bromo (the most popular) and the Tengger people who inhabit the area.
Mount Semeru also known as Mahameru (“Great Mountain”), is one of Indonesia’s most active volcanoes. What stands out most about this mountain is the fact that it erupts periodically (and very reliably). Every 20min or so, the volcano belches out a huge cloud of steam and smoke, sometimes interspersed with ash and stones. Climbing Mount Semeru requires some planning and a permit from the national park authority. The mountain is often closed due to its highly active nature.
Mount Bromo (2,329m) is easily recognized as the entire top has been blown off and the crater inside constantly belches white sulfurous smoke. It sits inside the massive Tengger caldera. with a diameter of approximately 10km, surrounded by the Laut Pasir (Sea of Sand) of fine volcanic sand. The overall effect is unsettlingly unearthly, especially when compared to the lush green valleys all around the caldera.
The major access point is Cemoro Lawang (also Cemara Lawang or Cemora Lawang – blame the East Javanese accent!) at the northeastern edge of the caldera, but there are also trails from Tosari (northwest) and Ngadas (southwest). The village of Ngadisari, on the road from Probolinggo about 5.5km before Cemoro Lawang, marks the entrance to the national park. Both Cemoro Lawang and Ngadisari are rather picturesque, with brightly-painted houses and flower beds outside.
Source : http://www.wikitravel.org