Nature documentaries have been scientifically proven to boost happiness and bring smiles to nearly all who watch them. As a recent episode of "Blue Planet II" made clear, they're also extremely good at doing the opposite.

The BBC nature series, which took four years and 125 expeditions across 39 countries to create, left viewers heartbroken recently after focusing on a mother pilot whale found clinging desperately to her dead calf. As narrator David Attenborough explained, the deceased calf may have become poisoned by its mother's own contaminated milk.

"In top predators like these, industrial chemicals can build up to lethal levels," he said. "And plastic could be part of the problem. As plastic breaks down, it combines with these other pollutants that are consumed by vast numbers of marine creatures."

As shown in the video below, Attenborough goes on to share how the grief of the mother has also extended to the rest of the pod. A 2015 study on the brains of long-finned pilot whales found that they had more neocortical neurons (responsible for sensory perception, spatial reasoning and conscious thought, among other functions) than any mammal studied to date including humans.

After watching the episode, viewers took to social media to decry the growing crisis of ocean pollution, with many swearing off plastic products as a result. Even Michael Gove, the U.K.'s environmental secretary, was moved to take action.

Others, however, such as associate professor Malcolm Hudson, an expert on ocean pollution at Southampton University, questioned the lack of real evidence tying the calf's death to contaminated milk. The producers of the program later declared that no autopsy was done on the calf to confirm a cause of death.

An estimated 5.25 trillion pieces of plastic debris now call our oceans home, with some 269,000 tons afloat on the surface. Even in the Mariana Trench, the ocean's deepest point at 36,000 feet, 100 percent of the animals tested in a recent study were found to have ingested plastic.

"The fact that we found such extraordinary levels of these pollutants in one of the most remote and inaccessible habitats on Earth really brings home the long term, devastating impact that mankind is having on the planet," Newcastle University's Dr. Alan Jamieson, who led the study, said in a statement. "It's not a great legacy that we're leaving behind."

In 2015, the U.K. further cemented its position as a global leader in the fight against plastic pollution by introducing a plastic bag tax of 5-pence (about $.07 USD) to each sale. Since then, some 9 billion fewer bags have been distributed. The government this summer also announced a ban on plastic microbeads used in cosmetics and personal care products starting in June 2018. Both moves, Gove said in a speech earlier this year, are only the beginning of a larger plastic reduction scheme across multiple industries and products.

"There is more we can do to protect our oceans, so we will explore new methods of reducing the amount of plastic – in particular plastic bottles – entering our seas, improve incentives for reducing waste and litter, and review the penalties available to deal with polluters,” he added.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.