After gazing curiously at the stump for some moments, he squatted down, gripped it by a couple of protruding roots, and overturned it, causing it to tumble into the surf. He then edged back out of the others’ sight line to reveal what had been hidden beneath it: a vertical section of stout steel pipe, about a hand’s breadth in diameter, rising to the height of a person’s knee, topped with a flat disk of battered steel the size of a dinner plate. The pipe wasn’t rooted to the boulder itself. It was part of a longer object that extended out into the sea. The part above the waterline was lashed to spikes that had been driven into crevices in the rock, in a style that they had already learned to recognize as typical Digger improvisation.