The novel takes in various settings around and under Europe. It begins in Hamburg, Germany, where Axel lives with his uncle. From here the two set out to Sneffels, an extinct volcano in Iceland, and venture down its crater to begin their subterranean journey. Along with their guide Hans, they travel many miles underground, under Europe, encountering various wondrous places along the way such as a subterranean ocean and primeval forests. Finally, they emerge through an active volcano, Mount Etna, on the island of Stromboli, Italy.
It is only when they arrive back on the surface of the earth that Axel suddenly realises what an amazing journey they have been through.
Oh, what a journey! What a wonderful journey! We had gone in by one volcano and come out by another, and this other was more than three thousand miles from Sneffels, from the barren country of Iceland at the far limits of the inhabited world! The chances of our expedition had carried us into the heart of the most beautiful part of the world! (chapter 42)
The journey, then, has been an extremely varied one, encompassing vastly different regions on the surface as well as below ground.