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.
Dr Helen Czerski examines the hottest natural phenomena on the planet - lightning. Bolts of lightning five times hotter than the surface of the sun strike our planet over 3 million times every day - and yet we still know little about this deadly force of nature. Now, specialist photography is revealing how lightning travels through the air, high-speed cameras are unlocking the secrets of upward lightning that's triggered by our urban landscapes, and scientific expeditions are capturing rare images of intense electrical discharges over 80 kilometres wide.
In the wild North Atlantic, massive whale pods, giant turtles and monstrous jellyfish ride the Gulf Stream, a huge ocean current that becomes a migration superhighway and helps warm northern Europe. Meanwhile, fishermen battle for survival in mountainous seas as they try to reap the current's natural fertility.
Everyone loves giraffes, but what do we really know about them? Dr Julian Fennessy starts to reveal their secrets - the most important being that they are disappearing. In an urgent and daring mission, with a determined Ugandan team, he plans to round up 20 of the world's rarest giraffe to take across and beyond the mighty Nile River. The stakes are high, but if they succeed the reward will be a brighter future for an animal we have somehow overlooked.
Coral reefs are home to a quarter of all marine species. Survival in these undersea mega-cities is a challenge with many different solutions. A turtle heads to the reef's equivalent of a health spa - but she must use trickery to avoid the queue. A remarkable Grouper uses the fish equivalent of sign language to collaborate with an octopus, flushing their prey out of hiding holes. A metre-long, ferocious-jawed Bobbit Worm hides in its tunnel. Monocle Bream retaliate by squirting water to expose its sandy lair.