Which geographical feature is a level area that contains thick accumulations of sediment?

The geographical feature described is the abyssal plain, which is characterized by its flat, level terrain that lies at great depths on the ocean floor. Abyssal plains are formed by the accumulation of sediment, including organic material and particles from the water column above, that settles out over time. This sediment can be quite thick, often several kilometers deep in some areas, contributing to the even and expansive nature of these plains.

The abyssal plains represent one of the flattest and smoothest regions on Earth, contrasting sharply with the rugged terrain found in other oceanic features. This sedimentation process plays a crucial role in the formation of these plains, enabling them to maintain their characteristic level surface.