I wasn't quite as close to this eruption as I was to that of Anak Krakatau but I was impressed by it nonetheless.
This is Gunung Merapi near Yogyakarta in central Java. The southern coast of Java lies close to the junction of two tectonic plates. Such close proximity to a subduction zone (where one plate slides beneath the other) means the area is prone to earthquakes and volcanic eruptions.
I was at the ancient Buddhist temple of Borobodur when Gunung Merapi did its thing. Almost a thousand years before, the volcano buried a huge area of central Java in volcanic ash. Borobodur was left to the rainforest, which quickly smothered it. It remained hidden until Stanford Raffles rediscovered it in 1814. Restoration proceeded slowly. Borobodur and the neighbouring Hindu temples of Prambanan are now listed as World Heritage Sites.