The blue sky seemed to vanish in an instant, as wild, grey thunderclouds raced across the mountains. Rays of sunlight broke through in places, brightening spots in the forest like searchlights. There are few landscapes more dramatic than Yakushima, and few places with more weather; within seconds we were being pelted by our 12th rainstorm of the day. But none of this bothered Sachito Fujimoto, one of Honda's top engineers. "It's the perfect climate for us," he said with a grin, and we climbed into the dumpy little blue car he was testing. The Honda FCX isn't much to look...