How far below the surface of earth did Verne think his characters went?
I mean, the magma was still beneath them so it wasn’t even through the crust.
I bet we do have an underground ”ocean” or two in there someplace.
I saw those gigantic underground crystals in the caves in Mexico on NatGeo and water had to be involved.

channel.nationalgeographic.com
channel.nationalgeographic.com
channel.nationalgeographic.com