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There is a striking similarity in the wave patterns of the ash plume on the cover of the Sept. 13 issue (above) and those in the gas of the Perseus Cluster pictured in this article. Could it be that volcanoes produce sound waves we can’t hear but can see in the plume?

Paul Heins
Gainesville, Fla.

Volcanoes do indeed produce abundant pressure waves that have frequencies below the threshold of human hearing. In fact, because these pulses can travel thousands of kilometers, they can reveal distant, major eruptions .–S. Perkins