My, my, my, two in one week. This morning I left another minimal description of a book I read as a child. And it was answered within a couple of hours.
The book is The End of the Tunnel aka The Cave of Cornelius by Paul Capone. From the description on ABE's Book Sleuth's site:
"The Cave of Cornelius aka The End of the Tunnel by Paul Capon: Four children searching for a lost treasure of the Romans which they believe to lie somewhere in a cave near their home, stumble upon and into a secret world beneath the earth which is inhabited by descendants of the very Romans whose treasure they have been seeking. These people, with their debased Latin and their partly archaic and partly modern appurtenances, guard their secret and their habitat rigorously from the upper earth. Fortunately the children make contact with a contemporary who has long been a prisoner and who has the aid of a "native" girl. All escape by a complicated water and cave route which brings them out eventually in Paris - via the catacombs - with treasure and fame, leaving the secret of Sutteranea behind for good."
That's the good news. The bad: so far, I've only been able to find copies of this book for $300-$1500!
So, I'm relieved that I finally found the (I think) last sought-after-book-from-my-childhood. But I'll be jonesing for it till I finally hold it in my hands.
That might take a while.