Frozen Seas, The Blue Planet, BBC, Episode 4

This episode explores the unknown depths of the ocean. Over 60 of the sea is more than a mile deep and it forms the planet s most mysterious habitat. A sperm whale descends 1, 000 metres to look for food and is followed, with the Johnson Sealink submersible. On the way down, a number of unusual creatures are witnessed, such as transparent squid and jellies, whose photophores give pulsating displays of colour. In such dark places, both being able to see (or sense movement) and the means of quick concealment are equally desirable. To that end, some use bioluminescence as a means of detecting food or evading predators. A descent to the very bottom of the ocean some 4, 000 metres reveals life even at such cold temperatures, much of it new to science.