Dive 4 took remotely operated vehicle Deep Discoverer (D2) to a depth of 6,000 meters in the Mariana Trench, at a site dubbed "Hadal Ridge." This was the first deep dive of this leg of the expedition, with the goal of documenting the diversity of organisms and geology at this transition between abyssal and hadal zones. As D2 worked its way up the ridge, we got a glimpse of the complexity of the trench's inner wall. D2 set down on fine sediment covered with ripple marks and a few rocks – likely peridotite (mantle rock) and calcium carbonate. Above this, we observed tongues of talus consisting of a variety of pebble- to cobble-sized fragments of mixed composition that were dominated by carbonate. At a depth of 5,898 meters, D2 encountered a stratified outcrop of very light-colored material (possibly carbonate or serpentinite mudflow material). The outcrop was approximately 53 meters high and topped by darker "polymict" (multiple rock types) and loosely consolidated formation of what appeared to be serpentinite mud with numerous pebble-to cobbble-sized rock clasts imbedded in it. Toward the end of the dive, D2 traversed a series of knife-edge ridges and troughs exposing some stratified layers of light-colored material. The dive ended at 5,750 meters, concluding a fascinating dive where the observed seascape was unlike anything the participating scientists had ever seen under water. Although the fauna was sparse on this dive, we did observe brisingid seastars, carnivorous sponges, shrimp, amphipods, and at the very end of the dive, a sea cucumber and a fish–likely a cusk eel.