Marine biologist Dr. Craig O'Connell and Andy Casagrande travel up the Serena River in the rainforests of Costa Rica to try and find out why young bull sharks swim up the river, and how they avoid the 12-14' American crocodiles living there. O'Connell deduces the sharks smell the crocs and creates a first-of-its-kind croc scent-based bull shark repellant - and SHARK WEEK reveals a possible important discovery that could save human lives