The magnificent Bromo-Tengger-Semeru National Park (or Taman Negara in Indonesian) is a volcanic area with still a few active craters. The world heritage listed N.P. spans more than 50.000 ha and consists of a mountainous highland at least 1000m above sea level, making it cold during the night and chilly during the day, a welcome surprise after Indonesia's tropical climate. Highest mountain of the park and of the island is Gunung Semeru, a still active volcano. Almost every morning the volcano will blow some smokes. You can hike almost to the top of this mighty mountain, but it is supposed to be very difficult and it's far away from any passable road.
Easier to reach is the Tengger caldera on the north side of the park, probably formed after the collapse of an old volcano resembling the mighty Semeru. The caldera forms a depression in the highlands, with a width of almost 20km at places. The rocky rim rises almost 300m above the caldera floor. On the rim is the village of Cemoro Lawang. To reach this tiny village you will have to take a minibus or taxi from Probolinggo, 1.5 hours away at the foot of the mountains, and right on the Surabay to Bali road, at 4 hours from the Gilimanuk ferry to Bali. If you are staying in the north of Bali, the village of Cemoro Lawang can be reached in one day.
From journal The Most Populous Island in the World