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How do abyssal plains become so smooth and flat?

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World map highlighting major features of the ocean basins, including trenches, ridges, basins, and plateaus labeled throughout the Pacific, Atlantic, Indian, Southern, and Arctic Oceans. Continents are shown with shaded terrain, and prominent undersea features such as the Mid-Atlantic Ridge, Mariana Trench, and Kerguelen Plateau are marked across the map. — ocean basin Major features of the ocean basins.

Abyssal plains become remarkably smooth and flat through the accumulation of land-derived sediment that fills in and buries preexisting irregular topography, such as hills and valleys.1

This process is primarily driven by turbidity currents—dense, sediment-laden slurries that flow down continental slopes due to gravity. When these currents reach the abyssal depressions, they deposit layers of silt, sand, and gravel, effectively leveling the seafloor. Between these episodic events, fine-grained clay and the microscopic remains of marine organisms settle slowly through the water column, further smoothing the surface.1

In some areas, this sediment accumulation reaches an average thickness of one kilometre, though isolated volcanic hills may still rise above the plain if the burial process is incomplete. Ultimately, these plains function as vast "ponds of mud" that mask the rugged, mountainous terrain characteristic of the deep ocean basin.12