Story : A documentary from the Ushuaia Nature series returns in the fourth episode of Superquark: Crystal Worlds, nature hides secret places of great beauty, inaccessible to us, which require experience, courage and special equipment to be explored. Tonight, with the Ushuaia crew we will enter some of these worlds: inside the Earth's crust, among caves and tunnels and incredible crystals and long underground rivers. For this reason we will descend into the canyons of Mexico, and inside its caves to finally land on a tiny island in Colombia, and witness a spectacular dive into deep waters. On this small island we will meet a rare and fragile animal: the Glass Frog. Scientists and naturalists will accompany Nicolas Hulot on this intriguing journey.\nItalian excellence: at Superquark an out-of-weight and out-of-shape transport engineering company that is one of the four leading companies in the world. Following in the studio with Piero Angela and Paco Lanciano an experiment on the wheel.