The most active volcano in Indonesia is also its most sacred. It represents the vertical axis of the trilogy that unites the kraton of Yogyakarta to the Indian Ocean and to the mountain. At an altitude of 2 930m, Mt Merapi, or the "Mountain of Fire", is as fascinating as it is terrifying. Its eruptions regularly threaten the local population and villages and it can only be climbed with a guide and should always be approached seriously.