At depths of, generally, between 20 and 250 fathoms in the oceans, sonic and ultrasonic transmissions are frequently scattered by a layer which can be detected on the echo sounding trace, sometimes so strongly as to suggest a sea-bed echo. The cause of this layer, of which the depth has been observed to rise at sunset and sink at sunrise, is not precisely known, though it is generally thought to be biological. Investigations into the deep scattering layer (DSL), as it is called, are being conducted in many countries, principally in the United States, France and Great Britain, and a recent paper by Tchernia has set out the present state of knowledge on the subject.