The wording of your question is a bit difficult to understand. The entirety of the narrative of Journey to the Center of the Earth takes place over the course of a single descent in the direction of the center of the earth from a volcanic crater in Iceland. After several misadventures within the dark subterranean caverns, including near-death from thirst and separation from one another, three intrepid explorers, a scientist, his nephew, and his guide, make their way to a massive cavern beneath the earth. Inside this gas-lit expanse, a prehistoric ocean is waiting for them, and they set sail on an unforgiving, subterranean sea for an indeterminate amount of time.
Soon, they come across a cavern marked by Saknussemm, the explorer who passed through this gauntlet before the adventurers, which has evidently been blocked by a landslide since the latter's journey. Axel suggests that they make it through by blasting the rock to continue further toward the center. This is the closest apparent incident in the book that resembles a "second descent." However, what the adventurers find is that the "bottomless pit" that appears before them is in actuality a vocanic vent, and the three are eventually pushed back to the surface of the world by the building water and magma.