How Deep Is Your Love review – under the sea at Earth’s last frontier with the Psychedelic Elvis Worm

For the marine biologists in this whimsical eco-documentary, the answer is about 5000 metres down. In the lab, we watch them observing, describing, and classifying the bonkers-looking creatures that survive in the freezing-cold waters of the deep sea: blobby little sea pigs and alien glow-in-the-dark sea cucumbers. It seems 90% of life in the deep sea has yet to be named, explains film-maker Eleanor Mortimer in her quirky voiceover. It is the last frontier of exploration on Earth.

Mortimer accompanies a team of scientists on an expedition to the eastern Pacific. It takes the ship’s robot four hours to descend the four kilometres to the ocean floor where it trundles along, grabber claw at the ready. A biologist explains that 80% of what they bring up to the surface is new to science. And one of the film’s pleasures is watching the wonderstruck awe on their faces as they clap eyes on never-before-seen creatures. The process of acquiring a Latin name for a new discovery can take years, so in the meantime scientists give them excellent temporary names like the Headless Chicken Monster and the Psychedelic Elvis Worm.

In truth, it’s a niche-feeling film, gentle and playfully curious, but an environmental story lurks beneath: deep sea mining. The ocean floor is covered in nodules – lumps the size of baked potatoes – that are rich in metals and minerals. They take millions of years to form, and significantly less time to mine. We listen in to a frustrating debate at the International Seabed Authority about whether mining should be permitted. How high this new eco-worry should rank on the spreadsheet of doom and gloom is as yet unknown. The environmental impact of mining is not yet established. But add it to the list.