Challenger Deep, at the southern end of the Mariana Trench, is the deepest part of the ocean at about 10,935 metres
Background
The average depth of the ocean is about 3,682 metres (12,080 feet).
The deepest part of the ocean is called the Challenger Deep.
It lies beneath the western Pacific Ocean, in the southern end of the Mariana Trench.
The Mariana Trench runs several hundred kilometres southwest of the US territorial island of Guam.
Challenger Deep is approximately 10,935 metres (35,876 feet) deep.
It is named after HMS Challenger, whose crew first sounded the depths of the trench in 1875.
In plain words
Ocean trench Deep narrow seafloor depression — A long, deep depression in the seafloor; the Mariana Trench is one.
Sounding Measuring water depth — HMS Challenger's crew first sounded the depths of the trench.