Dive 15 was conducted along the deeper terrace of "Explorer Ridge," to continue exploring the geomorphology and biology of the complex structural high east of the volcanoes in the Marianas Trench Marine National Monument islands unit. Remotely operated vehicle Deep Discoverer (D2) rose along a fault-controlled wall with a dive track that covered about 300 meters of vertical relief on the wall, showing layered sedimentary rock. Throughout the dive, layers of ever-changing texture and degree of fracturing were observed. This was a coral-rich environment, with the most abundant taxa being chrysogorgiid octocorals and black corals. Metallogorgia sp. were also abundant, along with its typical commensal brittle star, Ophiocreas oedipus. Other cnidarians included a lyrate bamboo coral – likely a range extension, "rock pens," a small jellyfish, seapens, and large Iridigorgia corals. D2 also encountered an isopod – likely a Thylakogaster sp. – camouflaged with sediment that has never before been seen alive. Observed sponge fauna included several species of hexactinellids, thinly encrusting demosponges (white and blue), carnivorous sponges, and lobate species on a large piece of debris. Fish were sparse on this dive, but included a likely sorcerer eel, a halosaur, and a rattail.