From TARDIS Index File, the free Doctor Who reference.

Krakatoa is a volcanic island in the south Pacific Ocean. It erupted in 1883.

Sometime prior to his third incarnation, the Doctor witnessed the eruption. During this event, he also heard Primords scream for the first time. (DW: Inferno)

That same day, a sketch of the Doctor washed up on a beach on Sumatra. It eventually found its way into Clive Finch's collection of artifacts related to the Doctor. (DW: Rose)

The Doctor and Rose Tyler visited Indonesia, where they encountered the alien Chalderans. They were captured and hung upside down, but the Doctor used his sonic screwdriver to free them. They escaped in the TARDIS just as the volcano erupted, killing the Chalderans. (DWAM: Under the Volcano)

The eruption also caused a Xylok to be freed; this Xylok became Mr Smith many years later. (SJA: The Lost Boy)

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Krakatoa