UK broadcast 26 March 2006, 8.98 million viewers (33% audience share); US broadcast 22 April 2007
This episode explores "Planet Earth's final frontier": the world of caves. At a depth of 400 metres (1,300 ft), Mexico's Cave of Swallows is Earth's deepest pit cave freefall drop, allowing entry by BASE jumpers. Its volume could contain New York City's Empire State Building. Equally as impressive, we explore the otherworldly underwater caves of the Yucatán Peninsula. Divers appeared to be flying in water as clear as an air, as they give us a glimpse of the hundreds of kilometers of these caves which have already been mapped. Also featured is Borneo's Deer Cave and Gomantong Cave. Inhabitants of the former include three million wrinkle-lipped bats, which have deposited guano on to an enormous mound. In Gomantong Cave, guano is many metres high and is blanketed with hundreds of thousands of cockroaches and other invertebrates. Also depicted are eyeless, subterranean creatures, such as the Texas blind salamander and ("bizarrely") a species of crab. Carlsbad Caverns National Park is featured with its calcite formations. Mexico's Cueva de Villa Luz is also featured, with its flowing stream of sulphuric acid and snottite formations made of living bacteria. A fish species, the shortfin molly, has adapted to this habitat. The programme ends in New Mexico's Lechuguilla Cave (discovered in 1986) where sulphuric acid has produced unusually ornate, gypsum crystal formations. Planet Earth Diaries reveals how a camera team spent a month among the cockroaches on the guano mound in Gomantong Cave and describes the logistics required to photograph Lechuguilla. Permission for the latter took two years and local authorities are unlikely to allow another visit.
UK broadcast 2 April 2006, 9.23 million viewers (34% audience share); US broadcast 1 April 2007
This instalment features the harsh environment that covers one third of the Earth: the deserts. Due to Siberian winds, Mongolia's Gobi Desert reaches extremes of temperature like no other, ranging from -40°C to +50°C (-40°F to 122°F). It is home to the rare Bactrian camel, which eats snow to maintain its fluid level and must limit itself to 10 litres (2.6 U.S. gal; 2.2 imp gal) a day if it is not to prove fatal. Africa's Sahara is the size of the USA, and just one of its severe dust storms could cover the whole of Great Britain. While some creatures, such as the dromedary, take them in their stride, for others the only escape from such bombardments is to bury themselves in the sand. Few rocks can resist them either and the outcrops shown in Egypt's White Desert are being inexorably eroded. The biggest dunes (300 m or 1,000 ft high) are to be found in Namibia, while other deserts featured are Death Valley in California and Nevada, the Sonoran in Arizona, the deserts of Utah, all in the United States, the Atacama in Chile, and areas of the Australian outback. Animals are shown searching for food and surviving in such an unforgiving habitat: African elephants that walk up to 80 kilometres (50 mi) per day to find food; lions (hunting oryx); red kangaroos (which moisten their forelegs with saliva to keep cool); nocturnal fennec foxes, acrobatic flat lizards feeding on black flies, and duelling Nubian ibex. The final sequence illustrates one of nature's most fearsome spectacles: a billion-strong plague of desert locusts, destroying all vegetation in its path. Planet Earth Diaries explains how the hunt for the elusive Bactrian camels necessitated a two-month trek in Mongolia.
UK broadcast 5 November 2006, 6.37 million viewers (24% audience share); US broadcast 1 April 2007
The sixth programme looks at the regions of the Arctic and Antarctica. The latter contains 90% of the world's ice, and stays largely deserted until the spring, when visitors arrive to harvest its waters. Snow petrels take their place on nunataks and begin to court, but are preyed on by South Polar skuas. During summer, a pod of humpback whales hunt krill by creating a spiralling net of bubbles. The onset of winter sees the journey of emperor penguins to their breeding grounds, 160 kilometres (99 mi) inland. Their eggs transferred to the males for safekeeping, the females return to the ocean while their partners huddle into large groups to endure the extreme cold. At the northern end of the planet, Arctic residents include musk oxen, who are hunted by Arctic foxes and wolves. A female polar bear and her two cubs head off across the ice to look for food. As the sun melts the ice, a glimpse of the Earth's potential future reveals a male polar bear that is unable to find a firm footing anywhere and has to resort to swimming — which it cannot do indefinitely. Its desperate need to eat brings it to a colony of walrus. Although it attacks repeatedly, the herd is successful in evading it by returning to the sea. Wounded and unable to feed, the bear will not survive. Meanwhile, back in Antarctica, the eggs of the emperor penguins finally hatch. Planet Earth Diaries tells of the battle with the elements to obtain the penguin footage and of unwelcome visits from polar bears.[