abyssal plain
Oceanographic term for a flat or gently sloping area of the deep ocean floor, typically at depths of 2,200–5,500 meters.
Abyssal plains are flat seafloor features that extend beyond the continental shelf.
海溝 (trench) is a deep, narrow depression in the ocean floor, whereas 深海平原 is a broad, flat region.
大陸棚 (continental shelf) is the shallow, gently sloping area near continents, while 深海平原 lies in much deeper water.
Compound of 深海 (deep sea) and 平原 (plain), directly describing a flat area of the deep ocean floor.