The Caves of Night — John Christopher

Five people enter the Frohnberg caves, three men and two women. In the glare of the Austrian sunshine, the cool underground depths seem an attractive proposition â€" until the collapse of a cave wall blocks their return to the outside world. Faced with an unexplored warren of tunnels and caves, rivers and lakes, twisting and ramifying under the mountain range, they can only hope that there is an exit to be found on the other side.
For Cynthia, the journey through the dark labyrinths mirrors her own sense of guilt and confusion about the secret affair she has recently embarked upon. And whilst it is in some ways a comfort to share this possibly lethal ordeal with her lover Albrecht, only her husband Henry has the knowledge and experience that may lead them all back to safety.
But can even Henry's sang froid and expertise be enough, with the moment fast approaching when their food supplies will run out, and the batteries of their torches fail, leaving them to stumble blindly through the dark?