When something is unfathomed, it's so deep or far away that it hasn't been explored. There is a huge expanse of the ocean floor that remains unfathomed.
You can use the adjective unfathomed to describe quite literally places or objects that are too deep to reach or investigate. In writing, it's almost always the ocean or the sea that's described this way, since the deepest areas of these are still mostly unfathomed, even today. You can also use it figuratively: "I can't understand the unfathomed depths of his poetry." A fathom is a unit of measure mostly used for water.