Temperature layers in the ocean duct sound and enable very long range propagation.

Evidence

Refraction can also occur at the boundaries between liquids at different temperatures, such as water, and in some cases, the level of refraction can result in total internal reflection. This effect is sometimes used by submarines to hide from the sonar of other ships; it can also cause the sound to be ducted between two boundaries, and in these cases sound can cover large distances. It is thought that these mechanisms allow whales and dolphins to communicate over long distances in the ocean. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 339

Categorization

• QUESTION: Worth confirming which acoustic layers/ducts (e.g., SOFAR channel) make whale communication possible at these ranges.
• XREF: Connects to sonar masking and marine bioacoustics, and to the later chapter on sound in water/underwater acoustics likely in this book. — connects to: Ocean acidification physically amplifies how far sound travels in seawater — Almo Farina, Soundscape Ecology: Principles,…, 1332; Each marine noise source occupies a characteristic frequency band at a measurable distance — Almo Farina, Soundscape Ecology: Principles,…, 1331
• CANDY: Submarines hiding from sonar via temperature refraction is a striking, easily-retold fact.