Krakatoa, or Krakatau, is a caldera volcano. Calderas are depressions formed when a volcano collapses into itself. The result is a large crater surrounded by steep cliffs and possibly filled with lakes.
The ancient volcano of Krakatau erupted and collapsed in 416 A.D., producing a caldera of three volcanoes, Rakata, Danan and Perbuwatan. The infamous eruption of 1883, which resulted in the deaths of 36,000 people, destroyed Danan and Perbuwatan. A new cone formed between the destroyed volcanoes, which is known as Anak Krakatau. This volcano has had small eruptions since 1927. The entire group of volcanoes is regularly referred to as Krakatau.