Though popular, Kimura's claims about Yonaguni are disputed. Robert Schoch, a geologist at the University of Boston who has dived at Yonaguni many times, thinks the formations are mostly natural. They are made of bedrock, rather than built with separate blocks, and Schoch points out that the rock is sedimentary, with horizontal layers that break along parallel lines as they erode. The region's tectonic activity also splits the rock along vertical fault lines. So the strong currents that sweep the area would erode rock along these lines, carving out platforms and steps, he says. "You get a regular blocky structure quite naturally."