Really it is impossible to say.
"What is this place ?" I asked, clinging to the others and shrinking back in alarm from its sheer edge and bottomless depth, for that this was enormous we could see by the shaft of light which flowed downwards farther than the eye could follow.
"It is a vent up and down which air passes from and to the central hollows of the earth," Yva answered.

"Doubtless in the beginning through it travelled that mighty force which blew out these caves in the heated rocks, as the craftsman blows out glass." "I understand," said Bastin.

"Just like one blows out a bubble on a pipe, only on a larger scale.

Well, it is very interesting, but I have seen enough of it.

Also I am afraid of being blown away." "I fear that you must see more," answered Yva with a smile, "since we are about to descend this pit." "Do you mean that we are to go down that hole, and if so, how?
I don't see any lift, or moving staircase, or anything of that sort." "Easily and safely enough, Bastin.