Fish dominate the planet's waters through their astonishing variety of shape and behaviour. The beautiful weedy sea dragon looks like a creature from a fairytale, and the male protects their eggs by carrying them on his tail for months. The sarcastic fringehead, meanwhile, appears to turn its head inside out when it fights. Slow-motion cameras show the flying fish gliding through the air like a flock of birds and capture the world's fastest swimmer, the sailfish, plucking sardines from a shoal at 70 mph. And the tiny Hawaiian goby undertakes one of nature's most daunting journeys, climbing a massive waterfall to find safe pools for breeding.

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.