I do not know very much about the language and am doing research for a story that I am writing. I intend to use Fuka-sa Fumei as the name of a submarine. I apologize if I'm breaking any rules or codes of conduct for this site. Thanks in advance for any help or advice you can give.
The direct translation is unknown depths.
- 深さ -> depth
- 不明 -> unknown
The depth is unknown in the sense "you don't know how deep something is".
"深さ不明" implies we don't know how deep the sea under the ground level is if you are talking about the depth of the sea. If your submarine intends to go unexplored depth of the sea like James Cameron's submarine"Deepsea", "未到達の深さ" can be close. Probably abyss still works for the translation, too.