Pura Besakih is the "Mother Temple" of Bali.  It is not one temple, but an ensemble of many temples.  It is the most important and most sacred of all the temples in Bali.  

The history of Pura Besakih goes back to the 8th century.  It is located in close proximity to Mt. Agung, the highest mountain (a volcano) and the most sacred mountain on the island of Bali.  Besakih was initially a terraced sanctuary for worshiping the god of the volcano Gunung Agung.  Work on this complex continued for over 1,000 years, to its present size of about 30 temples and hundreds of shrines.  

The god of the volcano was REALLY angry in 1963.  We don't know exactly why the god was so angry.  He was perhaps angry because a very important  temple festival (Ekadasa Rudra, a sacrifice held only once a century) was performed on the wrong date.  Others say that the God of the volcano needed a sacrifice of human victims in order to be appeased.  The fact is that after 120 years of laying dormant, in 1963, the volcano erupted violently, killing thousands and destroying many villages.  Miraculously, the ensemble at Pura Besakih was spared.

This view is from inside one of the temples.  Notice the multi-tiered meru in the background.  The decorations (white cloth and yellow and white umbrellas) indicate indicate a festival in progress at this temple.