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Episode Five: The Great Flood

At the peak of the dry season in the Kalahari Desert herds of elephants trek towards a life-saving event.

These resourceful elephants reveal remarkable new behaviour as they try to make the most of the stagnant pools, arid woodlands and waterholes guarded by lion prides.

However, their fortunes change dramatically with the annual flooding of Botswana's Okavango Delta, which turns 4,000 square miles of desert into a maze of lagoons, islands and swamps.

As millions of animals are drawn to this wetland, including great herds of famished elephants and buffalo, the scene is set for some of the greatest clashes in nature.

Using a range of high definition filming techniques over two years, Nature's Great Events follows their fortunes.

A herd of the elephants look to be on a suicide mission as they head from the woodlands of northern Botswana to the parched sands of the delta – a place that appears to hold little food and water.

Incredibly, the experienced matriarchs time their arrival at the delta to coincide with the great flood, when water and lush grasses are abundant.

Nature's Great Events follows these resourceful elephants on their journey, revealing new and remarkable behaviour.

They film the way they use their trunks to siphon clean water from the surface layers of a stagnant pool, while avoiding stirring up the muddy sediment on the bottom with their feet.

Bull hippos also converge on prime territories formed by the rising flood water. They assert their dominance and gain mating rights with the females.

The explosive force of these two-tonne giants is captured on film as two big bulls do bloody battle – at times being lifted out of the water by their rival.

Lechwe swamp deer, zebra, giraffe, crocodiles and numerous fish invade the wetlands.

Countless thousands of birds also arrive in the delta, to enjoy the seasonal bounty during this time of abundance.

And, in a phenomenon never before filmed in the Okavango, thousands of dragonflies appear – seemingly from nowhere – within minutes of the flood arrival, mating and laying eggs.

As the flood finally reaches its peak, elephants and buffalo, near the end of their epic trek across the desert, face the final gauntlet of a hungry pride of lions.

The producer is Peter Bassett.

Nature's Great Events Diary – Mission Impassable

Filming in the Okavango requires a special type of film-maker, and few have more experience of working there than cameraman Mike Holding.

But when he was tasked with filming the front of the advancing flood, the team very quickly realised that this would be no mean feat.

Even finding a suitable area to film in a wilderness the size of Wales required Mike to take to the skies, using his plane to get a bird's eye view of the delta.

Once in the air, he could quickly locate a patch of sand that would soon be inundated by the flood.

Laying a GPS trail for the ground crew to follow with all the filming equipment, it took Mike just 20 minutes to fly the 25 miles back to camp – but it would take the crew three days to cover the same distance on the ground.

It would now be a race against time for the crew to get to the filming area before the flood. As the Okavango Delta is one of the most challenging places in the world to take a vehicle, they had to use specially-designed swamp trucks that can travel in up to seven feet of water.

Travelling in two vehicles meant that, when one got stuck, the other could come to the rescue. Mechanical failures and punctures were also an every day event.

The crew had to rely on their heavily improvised "bush mechanic" skills to get the trucks back on track and often changed tyres in four feet of water. Eventually, the team managed to get themselves and all their equipment into position to film the flood.

By arriving in exactly the right place at exactly the right time, the crew were able to use newly-designed high definition camera systems (such as a jib arm mounted onto the back of a filming vehicle and a fully motion-controlled macro-camera rig) to film remarkable new perspectives of the arriving flood.

Their struggle was worth it when, within minutes of the flood arrival, thousands of dragonflies appeared and began mating and laying eggs just centimetres away from the front of the advancing water.

Episode Six: The Great Feast

In the north-east Pacific, along the coastal waters of Alaska and British Columbia, the arrival of the summer sun triggers an explosion of plant life greater in scale than even the Amazon rainforest.

The feast draws in huge amounts of wildlife – including billions of herring; Steller's sea lions, who race against time to make the most of the abundant fish before the harsh winter closes in; and humpback whales that migrate all the way from Hawaii.

Remarkably, the basis of all this life is something so small that it's hardly visible to the naked eye. The sun sparks the growth of this phytoplankton – the microscopic floating plants that are the basis of all life here.

And both whales and sea lions have to overcome many obstacles before they can take advantage of the feast.

The humpback whales face a voyage of three months from Hawaii and the loss of a third of their bodyweight before they can feed again.

And, despite being the largest in the world, the Steller's sea lions are threatened by predators and ferocious seas. The mothers are land-bound as they have their pups and so cannot feed.

Nature's Great Events films a heart-rending scene where a mother loses her pup in a violent summer storm.

A struggle for life between a killer whale and a one tonne sea lion male is also filmed for the first time above and below water.

As well as the summer sun, the other key element that underpins the massive plankton bloom is the coastal landscape.

The complex coastline of islands, inlets and channels interrupts ocean currents and so brings nutrients to the surface, fuelling the plankton bloom.

In late summer the bloom is at its height and vast shoals of herring gather to feed.

Murres, small diving birds, round up a ball of herring and dart in to pick off fish. The underwater images show these graceful birds "flying" around the bait balls.

And, after a 6,000 mile round trip to Hawaii, the whales have returned and remarkable underwater footage reveals how one whale scoops up the entire ball of herring in one huge mouthful.

When a dozen whales converge, they employ the ultimate co-operative way to harvest the riches of the seas – through bubble net feeding.

One whale blows a ring of bubbles to engulf the shoal of herring and then they charge in.

Filmed from the surface, underwater and the air, Nature's Great Events reveals how these giant hunters can catch a tonne of fish every day.

The producer is Hugh Pearson.

Nature's Great Events Diary – Swallowed By A Whale

Filming The Great Feast underwater was the most difficult challenge for the team, but it was to provide some revealing new behaviour.

The strong currents and thick plankton bloom that make Alaskan and British Columbian waters so rich made the task of filming the herring bait balls a tough challenge.

The team had to dive in waters that experience some of the strongest currents in the world, as well as tackle an unusual safety issue – the possibility of being swallowed by a humpback whale!

Shane Moore and David Reichert were trying to film the herring bait balls – but they weren't the only ones showing an interest in these gatherings of fish.

The crew decided to turn their attentions to filming sea lions. The cheeky sea lions were a little too friendly, playing to the camera and nipping the divers.

But the crew needed to get more shots of the bait balls so, as the next strong tides brought up more fish, they tried once more.

As well as getting some stunning footage of the diving birds feeding on the herring, the unexpected actually happened: a 30 tonne whale roared through within feet of David and Shane, engulfing the entire bait ball.

Fortunately, the cameramen were unharmed and captured amazing and unique shots as well as an unforgettable experience.