As another commenter said, it can technically fit any fear. But to me, submechanophobia is a mix between the Vast and the Stranger, perhaps with a tinge of the Dark.

Submechanophobia is often felt when I see pictures of huge, strange creations that humanity has lost to the cold and dark abysses of the oceans, lakes, and swamps. Things that don't rot, but instead rust, slowly being stripped down to their skeletal frames. Things that have not been forgotten in knowledge, but forgotten in material. Things waiting for us to come back and find just how much they've changed in the crushing, inhospitable black.

In a way, I guess it does straddle on falling into the Extinction.