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The Indian Ocean is the third largest body of water on Earth at more than 6,000 miles wide and covering 13% of the world's surface. It is home to 5,000 species of fish, many of which only exist in the Indian Ocean. But it is also an ocean under threat from global issues such as over-fishing and climate change, which make this an ocean on the edge.

The tropical Indian Ocean is home to the Spice Islands. Characterised by beautiful sandy beaches, fringing coral reefs and coastal mangrove forests, this is a vibrant nursery area for marine life, such as whale sharks, crabs, seahorses and a great variety of fish. The coastal areas are where humans have the most direct impact on the ocean's resources.

The Cave of Swallows in Mexico is a 400m vertical shaft, deep enough to engulf the Empire State Building. The Lechuguilla cave system in the USA is 193km long and 500m deep with astonishing crystal formations hanging from its chambers. Although often overlooked, caves are remarkable habitats with equally bizarre wildlife. Cave angel fish cling to the walls behind cave waterfalls with microscopic hooks on their flattened fins. Cave swiftlets navigate by echo-location and build nests out of saliva. The Texas cave salamander has neither eyes nor pigment. Unique access to a hidden world of stalactites, stalagmites, snotites and troglodytes brings a wealth of surprises.

A dramatic journey along the coasts of South America, from the Falklands to the tropical Galapagos. The cold currents create rich fisheries, and support such diverse marine life as dolphins, killer whales, sea lions and albatross. But the most biquitous creatures are the penguins, which inhabit surprising locations - lush forest, arid desert and even equatorial areas.

This series illuminates unique and bizarre evolutionary journeys that have brought forth some of the world’s most impressive animals. We unearth a 50-million-year-old mystery mammal sought out a new source of food in the water, starting a spectacular evolutionary journey so that today it looks like a fish. How did a creature built for land become master of the ocean?

In Bhutan, Palin finds himself back in the land of yaks for a last look at the high Himalaya. Trekking to Chomolhari base camp he meets a nomad with a penchant for yak songs before heading down to Paro to witness the Buddhist festival or Tsechu. In a bar in Thimphu, he discusses reincarnation and the pursuit of happiness with Benji and Khendum, two of the king's cousins, and en route to Bangladesh is taken by Benji to see the rare black-neck cranes. On his journey south through Bangladesh, Michael visits the ship-breaking beaches of Chittagong and grid-locked Dhaka. He meets a man who made a fortune in Birmingham in the poultry business, and a woman who lends money only to women. On a 1920s paddle steamer he is serenaded with the words of Bengal's Shakespeare, and he completes his epic Himalayan journey aboard a fishing boat that carries him out into the Bay of Bengal and a westering sun.