In Chapter 34, why was Axel terrified and what did his source of fear turn out to be?

The voyage is once again dull and unchanging, until the men see a massive creature with a blowhole in the distance. This animal is stunningly large, massive beyond all comprehension. Finally, though, they realize that they have actually spotted an island and laugh in relief. The blowhole is a geyser, spewing out water of intense heat. Lidenbrock and his companions have covered 1,550 miles from Iceland and are now under Britain.

After a brief sojourn on the island, the explorers decide that it is time to depart again.