'Blue Planet 2' may have finished airing on the BBC, but it's just got a new lease of life through iPlayer. The Beeb has given it a 4K makeover, making it available in UHD (aka 4K) and HDR.
4K has four times as many pixels as high definition, making for much sharper image quality. HDR stands for high dynamic range – a technology that increases the different between the light and dark parts of the image, with more gradual 'steps' in between. This gives greater depth, making for a more lifelike picture.
To see the difference, you'll need a TV compatible with 4K and HDR technologies.
The BBC has published a full list of compatible tellies here.
If you have a supported TV, you'll see a special 4K-specific screen when you open the iPlayer app.
According to the Beeb, its Hybrid Log Gamma (HLG) format of HDR will make viewers feel like they're actually there in the deep blue alongside David Attenborough and the crew.
"Due to the way HLG represents light, people can enjoy the most natural colours possible on their televisions at home, as if they were in the sea alongside the camera crew," the corporation writes in a blog post.
"It's also a significant step towards bringing more HDR programmes to more people, as it meets the complex needs of TV broadcasters, especially with live broadcasts."