Michael Dahl 
Pit of No Return [PDF ebook] 

Young Quom Erro and his human friend Zak Nine are in trouble. They’re stuck in Planet Alcatraz’s most infamous jail cell: a huge metal cylinder that’s sunk into the ground but open to the sky. The boys are certain they can climb out, but they quickly discover the cunning secret of the pit. When they make too many movements, the floor sinks and the pit grows even deeper. Will the boys manage to climb out or will their hopes dwindle as the circle of sky overhead shrinks to a distant dot?