a cavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone. A woman visits the same beach year after year, and begins to notice a change to a large wall of stone. Where the water touches the rock, she notices that the rock shows signs of being dissolved. After two year, there is an indent in the rock. After fifty years, the woman discovers that the rock ['is now an underground chamber', 'is now solid gold', 'is now about to explode', 'is now much bigger']