Solitary is the second book of the Furnace series, written by Alexander Gordon Smith.
After attempting to escape once in the kitchen by blowing up gas-filled bombs, Alex Sawyer,Zee Hatcher, Toby Merchant, and Gary Owens charge into the abyss. They fall a great height; splashing onto the rocks luckily below. Alex almost drowns as well as Zee and Gary, but luckily, Gary pulls him up before he can drown. Toby is revealed to have drowned in the black water. The now-trio travel among the rocks; hearing the noise of monsturous creatures (which are later revealed to be dogs). They climb up a wedge in the stone wall, which is too high for the dog. Gary later finds an underground tunnel in which the boys burrow into. They follow for what seems to be hours, according to Alex, before reaching flat land again. There, the warden waits for them. They see Gary under his foot, strawn out and lifeless. Later, the warden announces that he will put Zee and Alex into a more secured cell.
He later shows them to what is described as a hollow burrow even farther underneath Furnace.