Read the passage below. Never the thing we might have done. Here below out feet is a world. Think of what that world must be! Think of that machine we saw, and the lid and the shaft! They were just remote outlying things, and those creatures we have seen and fought with no more than ignorant peasants, dwellers in the outskirts, yokels and labourers half akin to brutes. Down below! Caverns beneath caverns, tunnels, structures, ways… It must open out, and be greater and wider and more populous as one descends, and vanished like the creatures of a dream!
The passage above could be used to support a theme about:

the unimportance of material wealth
man’s fear of the unknown
man’s quest for knowledge
the shortness of our time on earth