From ? library in the United States. Red hardback cover…no jacket. College or high school kids on trip get lost underground/in caves and encounter literal demons…i.e. red skinned….within vast huge cavern…lava hot.
Stump the Bookseller
New format, same book stumper solving.
Perhaps have a look at Jostein Gaarder’s books? The name would fit, but not sure if any of his works are a clear match.