Five days and 116 years ago, a small island in the Sunda Straight between Java and Sumatra exploded.

They heard Krakatau go bang in Perth, Australia. And 4,600 km to the west they heard it too, on Rodrigues Island in the middle of the Indian Ocean. The airwave it created hit Bogota, Colombia, on the other side of the world 19 hours later. It then bounced, back and forth, seven times. Krakatau’s 40-meter-tall tsunami killed 40,000 Javans and Sumatrans, drowned one person in distant Ceylon and hit Le Havre, France, 32 hours later. (By that time, the killer wave was just 1 cm in height.) Sunsets around the world were extravagant with light flamed by floating ash for weeks afterward.

Krakatau was not the biggest volcanic eruption in human history. In 1815, the Tambura eruption on Sumbawa Island (also Indonesia) lifted “five times the rock and ash,” according to Harvard biologist E.O. Wilson’s magnificent book, “The Biodiversity of Life.” And 75,000 years ago, an eruption in central Sumatra (Indonesia again!) punched out an impressive 1,000 cubic km of solid matter. There’s a lake there now.

Krakatau was, nevertheless, a big one. The captain of a British ship lying 222 km south that day reported encountering “carcasses of animals including even those of tigers . . . besides enormous trunks of trees borne along by the current.”

When the ash settled, Krakatau, formerly the size of Manhattan, had largely ceased to exist. What remained was Rakata, a completely lifeless chunk of rock at the southern end of the former island. And Rakata presented the scientific world with a unique opportunity.

This sterilized environment offered early zoologists an unrivaled “living laboratory” in which to study the recolonizing process of life. The ship’s naturalist on a French vessel that searched Rakata for life forms nine months after the eruption, found just one: a spider. Although the Frenchman was perplexed by the presence of this lonely colonist, scientists now believe the spider arrived by ballooning _ a process in which the spider spins a web and, like a kite, is wafted by the wind sometimes many thousands of meters in height and for distances of hundreds of kilometers.

Indeed, the earth’s atmosphere is full of balloonists and other insects caught by strong updrafts. They form an airborne drift, known as aeolian plankton, that settles here or there at the whim of the wind.

Subsequent scientific visits to Rakata recorded new arrivals. Birds and bats, naturally. There’s no mystery about how they colonize islands (although, interestingly, many forest bird species refuse to cross large areas of water). There were also more surprising immigrants: a reticulated python (which reaching lengths of 10 meters is the world’s largest snake species); geckoes; rats. All of them presumably swam the considerable distance from the mainland or rafted over on floating vegetation. Unlike the volcanic Galapagos Islands where scientists believe it took on average 80,000 years for each new species to became settled (even the giant tortoises had to float 1,600 km from Ecuador), Rakata soon became rather crowded. By 1930, there were 300 species of plants alone.

In 1999, as our speed boat sliced through calm water beneath the bluest skies, there was little indication that this island had ever been seared by burning gas and ash.

Vivid tropical greens completely smothered Rakata. Eagles spiraled overhead. It was as peaceful and thriving a chunk of biodiversity as one could wish.

The same could not be said of Anak Krakatau (“Child of Krakatau”) which burst out of the sea just east of Rakata in 1927 and has been erupting, on and off, ever since. The last burst of violent activity occurred between 1992 and 1995, claiming the life of an American visitor.

The recolonization of life on Anak Krakatau is dramatically obvious. A monitor lizard, disturbed by our arrival, padded away. These entrepreneurial omnivores manage to scrounge a living on dead marine life that washes ashore. Carnivorous grasshoppers whirred through the air. Flowering creepers snaked across the beach. For several hundred meters inland, tropical fir forest and giant pandanus palms swayed in the rising breeze.

Today, Anak Krakatau is at least 350 meters high, and growing. It’s a tough climb and it wants to twist your ankles. Wild sugar cane clumps mark the last inroads of vegetation; thereafter it’s pumice, ash, sulphurous vents, lava bombs and splendid desolation, arcing sharply up to the steaming summit crater. Looking down from the heights you can clearly see the battle between life, pushing up, and catastrophic forces pushing it back.

Visits to the volcano are best topped off with a refreshing but bizarre spot of snorkeling. There are the usual marine marvels: giant clams, vivid clownfish weaving unharmed through the stinging tentacles of their host anemones, fluttering wrasses and so on. The underwater reef world, as anyone knows who has been there, is far from silent. There is the faint grinding scrunch as parrotfish nip and crunch hard coral with their beaks. The occasional squeak or grunt. Off Rakata these sounds are augmented from time to time by a threatening, thudding grumble — almost a groan — as Anak Krakatau shifts uneasily.

Getting to the volcanoes presents no difficulties. It can be done as a day trip from Jakarta, three hours’ drive and one and a half speedboat hours away. A safer option is to stay overnight at one of the many sea front inns, though. Although the dry season sees calm waters, for most of the year the weather is fickle, as we discovered.

It was a hellish return trip. From a perfect, cloudless morning was born an afternoon of lowering clouds and fast, gusting winds that churned the Sunda Straight into violent life. The speedboat thudded across the angry waves like a sledge that has run out of snow.

Drenched, battered, shaken and stirred, my heart went out to the monitor lizard that had to swim through this sort of stuff before taking up precarious residence on some of the world’s most temperamental real estate.

But that’s life, I guess. Tough.

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