Vulcanologists describe Gunung Batur as a 'double caldera', ie one crater inside another. The outer crater is an oval about 14km long, with its western rim about 1500m above sea level. The inner is a classic volcano-shaped peak that reaches 1717m. Geological activity occurs regularly, and activity over the last decade has spawned several smaller cones on its western flank. There were major eruptions in 1917, 1926 and 1963.
One look at this other-worldly spectacle and you'll understand why people want to go through the many hassles and expenses of a trek. Note that the odds of clouds obscuring your reason for coming are greater from July to December, but at any time of year you should check conditions before committing to a trip, or even coming up the mountain.