Dive 20 was conducted at Subducting Guyot 2, a seamount in the process of being subducted under the Marianas Plate. Remotely operated vehicle Deep Discoverer (D2) recorded unexpectedly large angular rocks covered with thin sediment. The rocks showed columnar joints (cracks in the rock), fractured so recently that manganese coating had not had time to build up. Many rocks were "massive" looking in terms of texture, but some were clearly breccias with pebble- and cobble-sized rock fragments embedded in an enclosing matrix. It is likely that this dive took place on the deeper eastern extension of an already subducted seamount, meaning the eastern igneous interior of the edifice was exposed on this wall and in the boulders derived from it. All of this suggests a relatively recent faulting event. Among observed fauna was an unidentified hexactinellid sponge, several glass sponges (at least five different species), brisingida, sea stars, crinoids, holothurians, and a hooded sea slug. Cnidarians were rare, but one very large bamboo coral for this depth and part of the ocean was seen and collected. Although not abundant, arthropod diversity was well represented: barnacles, mysids, isopods, shrimp, and squat lobsters. An exciting discovery was an undescribed species of Pachycara (or "eelpout"), an abyssal fish genus of the family Zoarcidae.