Krakatoa Volcano, probably the most famous volcano in modern history, it exploded in 1883, the resulting sound and waves observed in countries halfway around the globe, allegedly disrupting the earths climate for several years. It is now a World Heritage Site, and thus worth a visit if you are in Indonesia.
wikipedia tells me this ...
With a Volcanic Explosivity Index (VEI) of 6, the eruption was equivalent to 200 megatons of TNT—about 13,000 times the yield of the Little Boy bomb (13 to 16 KT) that devastated Hiroshima, Japan during World War II and four times the yield of the Tsar Bomba (50 MT), the largest nuclear device ever detonated.
The series of islands above are the remnants of the Krakatoa that exploded in 1883, the grey smudge in the centre is Anak Kratatoa, (son of Krakatoa), like a phoenix rising from the ashes is growing anew, reportedly at a rate of 5 metres per year. The islands are the edges of the caldera, measuring about 7kms across.
We left from Kalianda, but you can also do the same sort of trips from Anyer, Cerita, or Tanjung Lasung, since Krakatoa is in the middle of the choppy Sunda Straits separating Sumatra from Java.
Of course, it aint cheap or easy. Your looking at 3.5 million rups ($350) for a boat for a day plus lunch, entry fee and guide, a speed boat will seat 4 people and take 2 hours each way or an older wooden fishing boat will seat up to 10 people and take 4 hours each way.
Our first sight of the Volcano, in fact, it is actually, Anak Krakatoa, (son of Krakatoa) was impressive, a belching, smokey grey smudge on the horizon. You can see the other islands, that are part of the caldera.