In some areas of the ocean PDR records show a reflecting surface a few fathoms below the bottom. Such horizons are observed only when the sounder is operated with a short (5-millisecond) ping length (in echo sounding the transmitted sound is called the ping, and its duration is called the ping length). When a long ping is used the first returning echo masks any subsequent echoes occurring less than about 10 fathoms after the first echo. To establish continuity of the lower horizon it is necessary to run the recorder without interruption, sending pings once a second. Since a faulty pinging circuit or some accident of geometry could conceivably send out two closely spaced pings, the supposed sub-bottom echoes must be carefully checked to make sure that they are not both bottom echoes from two closely spaced pings. If two pings were being sent out the second echo would always symmetrically underlie the bottom surface. If, however, the two surfaces show local variations, it can be safely concluded that the deeper one is a true sub-bottom echo. In order to observe sub-bottom echoes the sea floor should be reasonably smooth since in rugged relief side echoes and crossing "highlight" hyperbolas obscure any sub-bottom echoes which might occur. Sub-bottom echoes in the Gulf of Maine have been well described by Murray (1947). In local inshore areas prominent sub-bottom echoes recorded by unmodified or slightly modified standard echo sounders have been used to map basement rocks (Smith _et al._, 1952). In the deep sea, sub-bottom echoes or "penetration" are observed most frequently in the continental rise, oceanic rises, and the far edges of the abyssal plains. Penetration is rare on the open continental shelf and on the continental slope. As the depth increases, echoes are more difficult to obtain, so that records from different depths cannot be directly compared in reference to ease of penetration. It nevertheless seems to be true that sub-bottom echoes are rare or absent on PDR records from the parts of the abyssal plains closest to the continental margin. Penetration in the continental rise is common but frequently irregular and intermittent. One of the most persistent and uniform sub-bottom reflecting horizons observed occurs on the outer ridge east of the Bahamas (south of 30° N.) (Pl. 6). Records from the abyssal plain immediately adjacent to the abyssal hills (Pl. 13 Fig. 4) and from the flat-floored tongues in the abyssal hills (Pl. 10) reveal some of the deepest and strongest sub-bottom echoes. Good sub-bottom echoes are common in the Bermuda Plateau.