Underworld

Blue Morris and Wendy McTavonish are very close friends with a taste for adventure, both twelve-years old and keen to explore.
During the term holidays they decide upon several treks to the forest of Blackbutt, stories having been brought to air of the dreaded yowie, sightings less than a week old having been reported.
The two children have a nemesis in the form of a bully known as Andy.
Andy has two side-kicks, both of whom he commands so masterfully with unbreakable puppet strings. No matter what corner Blue and Wendy turn, no matter where they happen to go in search of adventure, the Bully Brothers are there to torment them with threats of violence.
During one particular visit to the forest of Blackbutt, Blue and Wendy are chased by the Bully Brothers. Blue soon finds himself riding for his life, close behind Wendy and upon his racer, when suddenly his bike losses its grip upon the track and he ends up, face down, in a reentrant. It is here that he comes across Hobart, a sixty-year-old man with a secret.
Later that night, Blue and Wendy find themselves in the forest once again.
They are drawn with mounting curiosity into a situation where they find themselves trapped within a cave, Hobart by their side and a special stone in their possession. The stone is a sphere known by the yowies as the Beacon of Gorke.
The threesome is transcended into another world, where the nights and days are set in permanent twilight, not a star to be seen. They travel towards lights in the distance and become entangled in their own destiny to return to Blackbutt, known by the molemen of Gorke as Upworld.
The molemen pledge to help the children return but in order for it to be a success they need to join in a safari, an expedition to the limits of the city's territory, to find a fungus with magical power enough to transform them back in time, to the instant that they transcended the shaft to Underworld.