What is the main underwater feature in the southern and southeastern Pacific Ocean?

The East Pacific Rise is indeed the main underwater feature in the southern and southeastern Pacific Ocean. It is a mid-ocean ridge that spans a significant portion of the Pacific Ocean, characterized by tectonic activity where the Pacific Plate is moving away from the North American Plate and the South American Plate. This geological feature is part of the global mid-ocean ridge system, which is the longest mountain range in the world and is formed by the process of seafloor spreading.

The East Pacific Rise is particularly noteworthy for its relatively fast rate of seafloor spreading, which can occur at up to 10 centimeters per year. This feature gives rise to several volcanic islands and seamounts in the region due to the active movement of magma from beneath the Earth's crust.

In contrast, other options do not pertain to this specific area: the Australia Ridge is located near Australia, the Mid-Atlantic Ridge is found in the Atlantic Ocean, and the Mid-Indian Ocean Ridge is in the Indian Ocean. Thus, they do not serve as the main underwater feature in the southern and southeastern Pacific Ocean, emphasizing the significance of the East Pacific Rise in this context.