Final answer:
The word 'grave' fosters a somber mood regarding the Avenger and adds tension to Captain Nemo's disposition, implying a significant ending and a respectful or regretful leaving behind.
Step-by-step explanation:
The word grave in the sentence 'Already we were ascending, fast leaving the grave of the old Avenger.' evokes a sense of somberness about the Avenger and builds tension around the way Captain Nemo feels. The use of 'grave' suggests a final resting place or the end of something significant, which can contribute to a mood of seriousness or solemnity. In the context of a sea voyage or adventure, it implies leaving behind the past, possibly with a tone of reverence or regret. This usage also reinforces the theme of human struggle against the vast and indifferent forces of nature, as seen in the description of the sea and the indifference of the landscape.