Diving the Mexican Underground - Deep underneath Mexico's Yucatan peninsula, underground rivers wind their way to the sea. In some places, the forest floor has fallen in, leaving holes in the ground to access these mysterious underwater caves. The holes, called cenotes, were considered openings to the underworld by the ancient Mayans. Jonathan teams up with an expert cave diver to explore these spooky underground rivers. Venomous Fish - While most fish are completely harmless to people, there are some species that are mildly to extremely venomous and can actually kill humans. Jonathan travels the world to meet some of the most venomous fish in the sea. Greenland Sharks - Jonathan travels to Canada in search of the Greenland shark, a huge, lethargic shark that lives in arctic waters. It takes days of effort, but finally, in dark, cold, murky water, Jonathan finally meets this elusive animal, and its nothing like he expected.