Typically referred to as the "abyss":
Per Google:
The ocean abyss (abyssal zone) is a vast, pitch-black, and freezing layer of the deep sea extending from 4,000 to 6,000 meters (13,000–20,000 feet) below the surface. Covering over 60% of the Earth's surface, this realm is defined by crushing pressures, the complete absence of sunlight, and highly specialized, often bioluminescent lifeforms.