One of the root causes of all this activity could be the age of the lithosphere, that ever-spreading, always-moving seafloor crust. “South of eastern Java,” Dr Dyment said, pointing to a brightly coloured map on his computer, “the lithosphere is about 120 million years old. South of western Java, it’s younger, maybe 80 million years old. But alongside Sumatra, the crust is much younger, as young as 45 million years old. And then, of course, on the other side of that, things start aging again.