They eventually reach Reykjavík, where they hire the Icelandic eider hunter Hans Bjelke to guide them on the long journey to the volcano. After an arduous climb to Snaefell’s summit, the trio locates the correct crater, and they descend and find the passage. When they reach a fork, Otto chooses the eastern tunnel, but after three days they enter a cavern in which the history of the Carboniferous Period is visible, and Otto realizes that he was mistaken. They return and head down the other tunnel. The adventurers exhaust their water supplies, but Hans locates a subterranean river, and they follow that thereafter. One day Axel takes a wrong turn and gets lost, but eventually an acoustic phenomenon allows him to speak to Otto and Hans, and he is able to rejoin them.