noun, の adjective
The most deep is the deepest.
最深 is a formal word used in geology to describe the deepest point of things like caves and oceans.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
300 meters at its deepest
The depth of the water is 300 meters at its deepest point.
In the Volcano of Death, we will head to the deepest part of the cave.
Sognefjord is the longest and deepest fjord in the world.