Deep inside the mountain, the cave opens into a lake. The water is so clear that the measured visibility exceeds ninety-eight meters — among the highest recorded for any body of water anywhere. Ryusendo Cave in the Iwate mountains is one of Japan's three great limestone caves, and the least visited of the three, because Iwaizumi Town is not on the way to anywhere. The transparency of the lake produces a color that has no equivalent in ordinary experience — a deep, still blue that intensifies as the light penetrates downward and the bottom recedes beyond sight. The limestone formations that frame it are secondary to this blue. Everything is secondary to this blue. Iwaizumi is a small town in the Northern Sanriku highlands, not well connected by public transport. Most visitors to Iwate go to Hiraizumi or Morioka. The people who come to Ryusendo come specifically for it, which means they come prepared to spend time with it. This is the correct approach. The cave rewards patience. The lake, seen for long enough, begins to seem less like a geological feature and more like a decision the earth made about what kind of beauty it wanted to keep to itself.