Critics have called Sir David Attenborough's latest series of Planet Earth "awe-inspiring" and "magnificent" but also "horrifying" and "sad".
The third instalment of the award-winning programme began on Sunday on BBC One, watched by 5.6m people.
The eight-part series shows animals around the world fighting for survival amid constant environmental change.
The Guardian described episode one as "yet more majestic TV" from the veteran broadcaster.
"This awe-inspiring series has a scale that is simply spectacular," wrote Rebecca Nicholson in a five-star review. "It is possible to watch and enjoy it purely for the astonishing footage - but it will horrify you too."
She added: "It should be alarming that, in the six years since Planet Earth last appeared on our screens, this third series finds itself in a darker mood."
The documentary, narrated by Sir David, 97, contains footage of the natural world - including shots from overhead drones and remotely operated deep-sea submersibles - gathered over five years across 43 countries.
The first episode is dedicated to coasts from Kent to South Africa, Mexico to Australia and beyond. It focuses on two cautionary tales; around the plights of the Caribbean flamingos on Mexico's Yucatán Peninsula, and the similarly endangered green turtles of Raine Island on the Great Barrier Reef.
We see old footage of a young Sir David visiting the same island more than 60 years earlier, and hear of the devastating impact human activity has had.
The Times' Carol Midgley, in a four-star review, said the new series "is magnificent, but it's a fast track to becoming really quite sad."
"I thought the footage of the desert lions paddling in the sea to catch cormorants mid-air in the pitch darkness was amazing," she wrote. "Miserably, so was the poor Caribbean flamingos having their entire nests wrecked by worsening storms (attributed to climate change), their chicks pathetically trying to scramble onto rocks as Attenborough's voice doomily explained: 'Soaked and cold [they] will soon perish unless they can get out of water. Some years no chicks survive.'"
The Telegraph's Ed Power wrote that Sir David "remains peerless when it comes describing the beauty - and fragility - of our planet".
Planet Earth III "packs the sort of dazzling visual punch of which Hollywood could only dream, with languid overhead shots of flapping flamingos and their young who struggle to survive in the freezing rain".
"At a time when there is so much uncertainty in the world, how enormously reassuring to know Attenborough is still on hand to share his passion with us," he concluded.
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This time around, the series has moved to an earlier time slot of 18:15 BST, a move Attenborough and the producers said would give children more opportunities to observe and understand the natural world.
"Children have an instinctive understanding about the way the world operates," Sir David said recently, while also issuing a warning about the perils of deforestation.
"The huge problem is the way we are gobbling up space, and have gobbled up space as though it belongs to us and nobody else.
"And the notion that you should actually have to restrain yourself in order to accommodate the natural world is not one which everybody feels."
He added: "We need to persuade people that it's quite a selfish thing to do because, apart from anything else, we depend upon the natural world and we had assumed that the natural world was inextinguishable for many, many years and no matter what we did, we could do what we like, because the natural world was always there.
"It is not always there, simply because we have now become such a dominant species in terms of numbers, we have come to realise that we have to live together and not just entirely on the terms that we choose."
Appraising his latest efforts, the Daily Mail's Christopher Stevens said: "Every moment of this opening episode was fascinating.
"It's impossible to pinpoint any Attenborough series from the past seven decades as 'the best'," he continued. "But Planet Earth III can certainly claim to be the most visually stunning."
In his five-star review, the I newspaper's Gerard Gilbert called the show "spectacular, eye-opening, awe-inspiring - and terrifying".
He stressed the latest series was "the closest David Attenborough has come to despair."
"Altogether, it was heartening to realise that there is still so much nature out there to discover - even if the underlying message seemed to be 'catch it while you can'."
Television
Nature
Climate change
David Attenborough