grammar - "Our subconscious" or "our subconsciousness"

Example:

There are many hidden things in the unreachable depth of our
subconscious/subconsciousness.

What's the most common choice? Is the first option ungrammatical?

Answer

There's a subtle difference between the two. Both of them are nouns, in addition to the word subconscious in the adjective form.

Subconscious¹ (adj.): feelings, desires etc are hidden in your mind and affect your behaviour, but you do not know that you
have them: [BrEn] a subconscious fear of failure

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Subconscious² (n.): the part of your mind that has thoughts and feelings you do not know about. [= unconscious]: anger buried deep in the subconscious

As you can notice, subconscious refers to a part of your brain, as some thing, where subconsciousness refers to an state of being. So your sentence would sound more grammatical with the former choice.

I also noticed that even subscription-only LDOCE does not have an entry for subconsciousness, where it mentions unconsciousness.

Checking Google NGrams, I'm being convinced that nowadays unconsciousness is more common than subconscious, however that won't affect your question or the sentence.