"In 1983 he found 21 places in Washington state with noise-free intervals of 15 minutes or more. By 2007 there were three. (One of them is Olympic National Park, which he is trying to save, and he will not reveal the names of the others, arguing that they are protected by their anonymity.) Whom can we blame? People, and planes. Hempton claims that, during daytime, the average noise-free interval in wilderness areas has shrunk to less than five minutes."