In the days of the Mayas, cenotes – deep, natural pits on the Yucatan peninsula - provided the only means of obtaining drinking water. But in the mythology of this advanced civilisation, these waterholes and caves were also the entrance to Xibalba – the underworld. All deceased were obliged to pass through Xibalba and wait there until they were called into heaven. It was a place in which one made sacrifices to the gods – objects of daily life, as well as bloody, human sacrifices. To this day, the relics of these acts are still in place preserved underwater for more than a thousand years. The entrance to the underworld begins at a dirty waterhole in the middle of the Mexican jungle overgrown and barely recognisable. But immediately after entry, a hall of breath-taking dimensions and beauty is revealed. As if they were sculptures, stalactites and stalagmites lend the underwater cave an almost sacred ambience. By now, one has succumbed to the fascination emanated by the world’s largest underwater cave system. We accompany four professional research divers to Yucatan – a team of specialists, able to squeeze their bodies through crevices and holes, barely larger than their bodies and who master dives that would push even the most experienced divers to their limits. With them, we penetrate worlds only a few people have ever ventured into. We encounter the remains of human victims, prehistoric fireplaces and primeval animal skeletons and undertake a dive, which takes us from the primeval forest to the open sea. We then enter Xibalba – the place of myths and the dead. In the days of the Mayas - a voyage of no return. Today, this is one of the greatest challenges one can face as a diver – the hidden world of the underwater caves of Yucatan – all filmed in 3D.

Covering one tenth of all species on the planet, Freshwater is a rich, diverse habitat filled with never-ending surprises. The fourth episode is set beneath the Yucatan Peninsula and spanning diverse locations like the rainforests of Costa Rica and the Kalahari Desert and presents a vivid exploration of how freshwater shapes life on Earth. The narrative journeys through the unique breeding rituals of gliding tree frogs in Costa Rica, the strategic hunting methods of mugger crocodiles in Sri Lanka, and the astonishing adaptations of wildlife in the Okavango Delta. It highlights the critical role of freshwater in supporting diverse ecosystems, from the teeming fish species in Lake Malawi to the perilous journey of gobies in Bioko.
The episode also addresses human impact, showcasing the extensive irrigation system in Pakistan and its unintended consequences on local wildlife, like the endangered Indus river dolphin. This captivating chapter not only showcases nature's marvels but also underscores the delicate balance between human needs and wildlife conservation.