Without a specific chapter to go on, I am making an educated guess that this question is asking about an event sequence that begins at the end of chapter 40. During this chapter, the men have returned to their raft, and they are sailing again. Unfortunately, their path is blocked by a really big rock. The men decide that blowing it up is a good solution.
Well then, the powder, an explosion! Let’s mine the obstacle and blow it up!
They successfully blow up the obstacle, and they quickly realize that their carelessness with the charges has now allowed the sea to rush through the opening.
The sea, seized with dizziness, had become nothing but one immense wave—on whose back the raft rose straight up.
The next two chapters narrate their voyage on that rushing water up through a series of vents. The water around them gets hotter and hotter, and the men (except Lidenbrock) are generally terrified of what will become of them.
Yes, if nothing stops us and if the shaft has a way out. But if it’s blocked, if the air gets more and more compressed by the pressure from the water column, we are about to be crushed to death!
Eventually the water is boiled away, and the men find themselves riding a column of magma. They are erupted out of a volcano, and they eventually figure out that they have been pushed out of Stromboli and see Etna in the distance.
Stromboli! What an effect this unforeseen name produced on my mind! We were in the middle of the Mediterranean, surrounded by that Aeolian Archipelago of mythological memory, in that ancient Strongyle where Aeolus held the winds and tempests on a chain! And those rounded blue hills to the east were the mountains of Calabria! And that volcano on the southern horizon was Etna, terrible Etna itself!