Answer:  When two Average Joe's from the 21st century find themselves in the 41st century where it's all good.  

We have all the natural resources we need, everybody has plenty of food and water. We found a cure for not only disease but also death.  War and crime both have been eliminated.  Working for a paycheck is unnecessary, leaving everyone to pursue self-actualization.  Sounds like a paradise, right?

"Ellis said . . . 'Maybe we can't see that Hollow World is paradise, because it's perfect but we're not.'

'Bullshit.'  Warren waved a calloused hand at him.

'How is that bullshit?'

'Because it's all wrong. Paradise isn't a lack of want. That's hell, brother . . .  This is the mistake everyone makes. Life is all about conflict. The pursuit of happiness – that's life, not the achievement.  Life is intended to be a battle, a struggle. God designed it that way. Think about it. Everything is always in constant conflict. Heat versus cold, light versus dark, . . . Every living thing in existence has to fight and kill to survive.  Who do you think did that? It's God, Ellis. God made the world like a cage match. You go in and you fight to win or die trying.'

'Survival of the fittest,' Ellis said, putting that piece in place . . . Warren had managed to succeed where centuries of scholars had failed – reconciling science and religion."

This story is really entertaining and also thought-provoking.  Ellis and Warren don't like everything they see, and being celebrities of a sort -- having come from the far past and also being distinctly different in appearance -- they have gained a certain amount of celebrity enabling them with power to change things.  They don't agree with each other on what should be changed and in which direction.  You can tell from the conversation repeated above that there is going to be conflict, between themselves and between them and the Hollow World inhabitants.

Sullivan does an excellent job on the world building and on characterization.  By the 41st century, we have moved underground, but this subterranean world is nothing like H. G. Wells' dark and mechanical habitat of the Morlocks.  Hollow World is full of abundance, beauty, light and music, good food and dancing.  

Everything is fine before the 21st century guys drop in, but their appearance and interference throws a big wrench in the mechanics of the world and disrupts the individual lives of the citizens.  

Along the the way to the revolution and the resolution, there is plenty of suspense, drama, humor, personal interaction, murder and explosions to make almost anyone happy.

Of course, my question is, WHY? do we all always think "my way is the best way" and take steps to make sure everyone falls in line. I believe it's a fault of major proportions.

 (And one I find myself guilty of way too often.  Stop it! with a face slap)