Science writer and explorer Gaia Vince visits Costa Rica to find out how this remarkable little republic has become 'the happiest, most sustainable country in the world'. Costa Rica runs its national grid on rivers and volcanoes and by 2021 it will be carbon neutral. It has no military, its rainforest is actually increasing, and it's home to more than 500,000 species. In this final episode Gaia goes horse riding with a 99-year-old cowboy, takes a canoe trip to meet the indigenous Bribri and witnesses an incredible natural phenomenon as thousands of olive ridley turtles arrive on a remote Costa Rican beach to lay their eggs: known as an 'arribada'.