A Channel in the Ocean

[A] Sound travels faster in warm water and faster again under high pressure. Near the surface the sea is warm; far below, it is cold but heavily compressed. Between the two lies a depth at which sound moves more slowly than it does either above or below. [B] A sound wave that strays upward from that layer enters faster water and is bent back down; one that strays downward is bent back up. The layer therefore behaves as a channel, holding sound inside it.

[C] The practical consequence is remarkable. A modest explosion set off in this layer can be detected thousands of kilometres away, because the energy that would ordinarily spread in all directions is confined to a horizontal sheet. [D] Navies recognised the possibilities immediately, and for several decades the channel was studied mainly for military reasons. Biologists arrived later, with a different question: several species of whale sing at frequencies that carry particularly well in it, and whether they exploit the channel deliberately is still argued about. The recordings that would settle the question are difficult to make and harder still to interpret.