Going down into these narrow flooded passageways is not for the faint-hearted but, with only an estimated one percent of the caves explored, it is an opportunity for the team to write themselves into cave diving history, by pushing further in than anyone has before. This is a challenge that tests even the most accomplished cave divers. Steve must face the terrors of being lost in an underwater silt cloud in a cave. But there is a bigger issue at stake. All life in the Yucatan depends on the fresh water in this network of caves, but it's being contaminated by human development. With every metre the team maps, it adds to the knowledge of the system, which, in turns, helps protect it for future generations.
What makes plants grow is a simple enough question, but the answer turns out to be one of the most complicated and fascinating stories in science and took over 300 years to unravel. Timothy Walker, director of the Oxford University Botanic Garden, reveals how the breakthroughs of Swedish botanist Carl Linnaeus, Chelsea gardener Phillip Miller and English naturalist John Ray created the science of botany. Between them these quirky, temperamental characters unlocked the mysteries of the plant kingdom and they began to glimpse a world where bigger, better and stronger plants could be created. Nurseryman Thomas Fairchild created the world's first artificial hybrid flower - an entirely new plant that didn't exist in nature. Today, botanists continue the search for new flowers, better crops and improved medicines to treat life-threatening diseases.
More than a billion people around the world commute into cities each day, and they are not alone. The world's wildlife is commuting too. A steady flow of animals journey in and out of cities to find food and shelter or to start a family. Leaving the wilderness they must overcome the unique challenges that the urban world throws at them to benefit from the opportunities on offer. This episode explores whether the secret to an animal's success in this fast-changing world is to keep one foot in the wild and one in the city, becoming a wild commuter. It seems that all over the world animals are finding that the city can offer opportunities that are harder to come by in the natural world. Some, like African penguins, whose population has plummeted by 80 per cent in the last 50 years, find shelter in the city. By nesting in Cape Town they are safer from predators, and with relatively easy access to their fishing grounds they have the best of both worlds. Many other animals commute into cities because they are filled with food. In St Lucia, South Africa, that includes hippos. Able to eat up to fifty kilograms of grass in a single sitting, they have developed a taste for the short, manicured lawns and come to town every night to dine out. St Lucia's human residents have learnt to give the hippos the space they need during their night-time raids. Black bears need to eat more than 20,000 calories a day to survive their six-month hibernation through winter, and using their acute sense of smell they can easily track down leftovers. In North America they come into towns and cities in search of food. Many animals displaced from their natural habitat are now using their wild skill set in the city to help fulfill their needs. Could this be the beginning of a new and very modern migration?
In this final episode, Iolo explores bird design - from their ability to fly to the way that their beak design, colour and camouflage enable them to live in the many habitats Wales has to offer. Using ultra-slow motion photography, Iolo looks at how garden birds have such control over take off and landing, and explains why fulmars are one of our most supreme fliers.