In May 1884, a group of schoolboys on a beach in Zanzibar came upon a large mass of pumice stone that had washed up at the tidemark. Evidently it had been floating in the sea for some time, as its bottom was crusted with barnacles and weed. Welded to its upper surface, they discovered, were dozens of skeletons, including humans, monkeys, and two big cats, probably Sumatran tigers.

It was a relic of the eruption of Krakatoa, which had taken place nine months earlier in the Dutch East Indies. The rock had floated 4,000 miles across the Indian Ocean to the east coast of Africa.

(From Simon Winchester’s Krakatoa, 2013.)