The title refers to the Berghof Sanatorium in Davos, in the Alps: the magic mountain is that place above the plain where time stops running, whoever goes up healthy gets sick, and whoever stays can no longer leave.
The spell of the place appears in several speeches. Joachim warns in chapter I that returning in three weeks "is an idea from down below." Dr. Behrens says in chapter IV that the mountain air is good against the illness and also good for it. Settembrini, after Walpurgis Night, reminds us that whoever has eaten the fruits of the kingdom of shadows belongs to it forever. And Hans Castorp, who came for three weeks, stays seven years.
In chapter VII, when the war breaks out, the narrator says that the thunder "blows the magic mountain apart" and throws the protagonist, like a sleepyhead, before the gates. The spell is broken only by an event from outside.
The book itself suggests a wider reach. The narrator places the story "in that world from before the Great War" and calls the thunder "historic," a shaking of the earth's foundations: the enchantment that holds a young man to whom his era gave no answer to "what for?" is also that of an entire era. On both levels, the Berghof is at once setting and illness, the place that holds through routine, through the air, and through the diagnosis.