In chapter four of Mary Shelley's Frankenstein, Victor Frankenstein is detailing (for Walton and through Walton's narrative) the toils associated with creating life, or reanimating life. The following quote speaks to the hidden nature of Victor's work.
One secret which I alone possessed was the hope to which I had dedicated myself; and the moon gazed on my midnight labours, while, with unrelaxed and breathless eagerness, I pursued nature to her hiding-places. Who shall conceive the horrors of my secret toil as I dabbled among the unhallowed damps of the grave or tortured the living animal to animate the lifeless clay?
This quote refers to two things. First, it refers to the fact that nature is always watching (important given the novel is Romantic). Second, it illustrates the fact that Victor knows what his is doing is wrong, or at least questionable. Given that Victor feels the necessity to conduct the majority of his work at night, hidden by the darkness, illustrates that he feels it is necessary to cloak his work with the dark. Parallelling the first idea, that nature is important, his pursuit of the moon (in its (personified) supremacy proves Victor to be in search of true forbidden knowledge.
One final idea the quote may re-illustrate is the fact that Victor truly recognizes the wrong in his experimentation. He states that he is both "unrelaxed" and "breathless[ly] eager." He would only be both tense and breathlessly eager if he knew what he was doing was truly wrong. Therefore, the seeking of the moon proves to be for two reasons: the desire to seek it out to possess its forbidden knowledge and in order to know where it is so that he may hide from it.
Victor is trying to discover how to create human life and how to discover immortality. Nature looks on (in the form of the moon) as he tries to figure out nature's mysteries (or hiding-places).
Victor is unrelaxed and breathless, which shows how nervous he is. He knows that he shouldn't be doing this, yet he is eager because he wants knowledge.