Yet these glassy waters hide an extraordinary world.
Headed by Paul Rose, ex-base commander of the British Antarctic Survey and dive trainer to the US navy, the Oceans team includes maritime archaeologist Lucy Blue, marine biologist and oceanographer Tooni Mahto, and conservationist Philippe Cousteau, grandson of the celebrated underwater pioneer Jacques.
Their aim was to seek out the hidden secrets of our oceans and, ultimately, to better understand how much we rely on these enigmatic and alien places.
But there were also signs of hope: coral that harbour a special heat-resistant algae that could also protect the other reefs of the world, and marine creatures with amazing abilities to adapt to their changing world.
They saw rare and endangered creatures and dived in alien marine environments: pitch-black waters turned purple by toxic bacteria, and eerie tannin-stained waters housing bizarre creatures.
They even dived into a throng of sharks to test out a new repellent – and were relieved to find it worked.
It is no surprise, then, that a new species was discovered by the team during almost every deepsea dive.
But there was more to find, including underwater caves that preserved the remains of lost civilisations and wrecks that spoke of ancient battles.
The mid-ocean ridge, a chain of mountains that runs through all the great seas, is 37,300 miles long, with an average height of 3,000 metres.
Together, the oceans make up an unimaginably vast environment wrapped around more than 70 per cent of the surface of the planet.
Four-fifths of all life on Earth is found beneath the waves, and scientists estimate a million new species are out there waiting to be discovered. Below the surface of the oceans there are mountains that would dwarf the Himalayas, waterfalls bigger than Niagara and more active volcanoes than anywhere else on the planet.
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