I remember someone lent me this book maybe around year 2000, at least not much earlier. I only remember the outline of the novel, all details are now lost to me. I read the book translated to Finnish.
It is a fantasy novel (in two parts I think) in which a boy for some reason enters a cave/tunnel of sorts, in which there is a lake. Diving into the lake gravity goes upside down and upon reaching the bottom he actually reaches another surface. He enters an underground realm inhabited by people who hid away from the Black Plague centuries ago. They have their warriors and there are some magical elements too - there is a temple/pyramid/whatever with a fearsome fire being inside.
There is some commotion and chaos erupts. I think the fire demon escapes and goes on a rampage. The boy escapes back to his own world but the warriors follow him into his home. For some reason they enter a car together and the boy, taking advantage of the warriors' ignorance of the modern world, brings the car to a halt very abruptly (he's the only one wearing a seatbelt), and escapes. I think finally he is caught, though.
Then it is revealed that the boy's father, who until now had been an absent figure, had actually once been the leader of this underground society. Now, in order to calm the mighty being of fire a sacrifice is needed, and the father stays to assume his responsibility as a leader, while the boy goes back home.
There may be mistakes and errors in what I just wrote as the memory is very faint already, but in case anyone recalls the name of this novel I'd greatly appreciate it.