In this landmark series for BBC One Wales filmed in Ultra High Definition (UHD), we dive into the natural wonders of the Welsh seas. The series features some of the largest creatures of the deep ocean, birds that can "fly" underwater and the tiniest marine life with the biggest personalities. In four beautifully-crafted episodes, we join the animals that live their lives on the edge in 'Coasts', jump into the mythical kingdom in 'The Shallow Seas' and head into the unknown for 'The Deep'. In the final episode we look at the environmental challenges facing the marine environments as we meet the 'Heroes of the Celtic Deep'.

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Narrated and presented by Dame Siân Phillips.

Music by Paul Mealor, performed by the National Orchestra and Chorus of Wales.