The world's most famous painting is hiding a massive secret! When an X-ray of the legendary Mona Mousa reveals a strange riddle, Geronimo Stilton and his sister Thea know they have a mystery to solve. But cracking this code will not be easy for a mild-mannered mouse who just wants a quiet life. Following a trail of hidden clues, Geronimo is pulled into a wild adventure deep beneath the bustling streets of New Mouse City. From dodging vicious octopuses to surviving hair-raising rollercoasters, every step brings them closer to a fabumouse treasure. But the dangerous underground is no place for a timid publisher, and Geronimo will have to summon all his courage to reach the end of the trail. Join your favorite frantic friend in the fifteenth book of the Geronimo Stilton series. Packed with non-stop action, this fast-paced mystery is perfect for readers who love a good puzzle. Can Geronimo decode the map and find the treasure, or will this underground expedition be his final chapter? Grab your magnifying glass and get ready to investigate!