The Mysterious Underground Men

A sapient rabbit named Mimio is born in an university lab experiment. However, Mimio thinks he is human.
So he escapes and finds refuge under John, an orphan kid scientist who is constructing a train capable
of penetrating the earth's core, thus allowing for safe transportation.

Initially, the train runs fine but it stops at the Earth's core.
When they get out, they are swept away by groundwater...

What lurks below the surface? Will Mimio and John survive?

Volume 253 of the Osamu Tezuka Complete Works