There's trouble Down Under as Matt, Jo and Dad set off on a new fossil-hunting adventure in the opal mines of Australia: the local miners seem to want them gone, Jo's showing signs of opal fever, and those claustrophobic tunnels don't look very safe either. When the trio's journey into the caves starts to go wrong, they start to suspect that there's something even more sinister afoot ...Can Matt conquer his fears and save the day?