Adventurer and naturalist Steve Backshall is on a mission to visit parts of the world yet to be explored. On this expedition, he is in the Arctic to kayak up the largest fjord in the world. It is a dangerous undertaking, only made possible because the sea ice has broken up early here following the warmest winter on record. It is a chance for Steve and his team of explorers to see what impact global warming could be having on this pristine wilderness. They face many threats, from perilous sea ice that could crush their boats to a terrifying encounter with a hungry polar bear.

Steve Backshall is visiting unexplored parts of the world. On this expedition, he attempts to summit an unclimbed mountain deep in Greenland’s Arctic wilderness. He faces many hazards - from rivers of freezing meltwater to quicksand and treacherous glacial ice. The Arctic is warming twice as fast as the rest of the planet which means Greenland’s ice sheet, over two kilometres thick in places, is melting. The climb lets Steve and his team look over an ancient habitat living on borrowed time.

Adventurer and naturalist Steve Backshall is on a quest to explore parts of the planet where no humans have been before. In this episode, he leads a team of elite explorers - including world leading underwater cave explorer Robbie Schmittner, former Royal Marine Aldo Kane and diving camera operator Katy Fraser - into the wilds of the Yucatan Peninsula in Mexico. Under the vast jungle here is the largest unexplored network of caves in the world, comprising thousands of kilometres of passageways. They are stunningly beautiful but incredibly dangerous - much of the system is under water. First the team must trek through tough, scorpion-infested jungle to camp alongside a giant sinkhole in the jungle floor - a dark gateway to an underworld full of nightmarish creatures. 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.

Explorer Steve Backshall travels deep into the Yucatan, Mexico, on the trail of the ancient Maya. His team – including Mexican archaeologist Guillermo de Anda, pioneering cave diver Robbie Schmittner and former Royal Marine Aldo Kane – want to explore secret caves the Maya called Xibalba, or the 'place of fear'. The ancient Maya believed the caves were a portal to a supernatural underworld, a place full of hellish terrors. It’s a mission that brings Steve face to face with the macabre secrets of this ancient civilisation. In the first cave they explore, the walls are covered in sinister handprints - some from children as young as two years old. But what happened to them? Abseiling in full dive kit, they lower themselves into the darkness of the next cavern. They come across a flooded graveyard full of human remains – all left as offerings to the gods. It’s a place where humans were probably sacrificed, and it's a chilling insight into the cruelty of the ancient Maya. But as they push deeper into more caves, they discover another side to the Maya. They witness incredible artefacts that add to our understanding of why the Maya risked their lives to travel so deep into this underworld. In the final cave, they discover a hidden underwater lake. Steve and Robbie don cave-diving kit to venture deeper into it than anyone has before.