That could describe quite a few different books, do you have any more details? Things that could help: how long ago you read it, whether it was a children's book or adult book, any character names or general info (for example, if you know that the main characters were 2 girls and a boy, or the man was a truck driver), was it a series, were they under a specific city such as London or New York?
I'll throw out some possibilities for now. 
Neverwhere by Neil Gaiman
Richard Mayhew's life is forever changed after he rescues a young girl named Door and finds himself living in a city of monsters, saints, murderers, and angels, and he must help Door on her mission to save this strange underworld kingdom from destruction.
When eleven-year-old Gregor and his two-year-old sister are pulled into a strange underground world, they trigger an epic battle involving men, bats, rats, cockroaches, and spiders while on a quest foretold by ancient prophecy.
When Will Burrows and his friend Chester embark on a quest to find Will's archaeologist father, who has inexplicably disappeared, they are led to a labyrinthine world underneath London, full of sinister inhabitants with evil intentions toward "Topsoilers" like Will and his father.
A fourteen-year-old boy who lives in the subterranean world beneath New York City meets a girl from above ground. When she visits him down below, she discovers his world is not what it appears to be.
