At sea level, the world’s biggest mountain range looks as flat as a pancake.

This ridge of peaks and valleys, which snakes tens of thousands of miles north to south, is longer than the Andes and the North American Cordillera and taller in places than the Himalayas. Yet few have ever seen it, let alone scaled its peaks, for the world’s biggest mountain range is submerged under thousands of metres of seawater.

We are in the very middle of the Atlantic Ocean onboard the RRS James Cook, a ship dedicated to scientific research (RRS stands for Royal Research Ship), and somewhere beneath our feet is the Mid-Atlantic Ridge. The sea is rough here, the waves more than 15 feet high; trade winds whip across the water at speeds of up to 50mph, tossing the ship and its crew this way and that.

When the clouds do part, there is no room for sunbathing, for the deck of this ship is festooned with cranes and heavy machinery, constantly in motion.

Among those on board is a special team of geologists on a special mission. By day they operate the machinery and when night falls they are to be found poring over long cores of rock, studying stones. These students of the earth have come to sea because here at the centre of the ocean is where new land forms.

It is happening right now: the North American tectonic plate is, as you read this, slowly parting ways from the Eurasian plate at a rate of about an inch a year: a gradual, inexorable process that kickstarted the break-up of the supercontinent Pangea 200 million years ago. And as these two continents part, all sorts of exciting geological activity steps in to fill the void: volcanoes, pillow lava, hot smoking towers of rock bubbling up from the depths.

The vast majority of this happens out of sight but for a sense of what we’re talking about, note that Iceland, land of volcanoes and lava and geysers, is one of the few terrestrial parts of the Mid-Atlantic Ridge.

Now imagine a lot more of that deep underwater, in a line that stretches all the way down the planet – a sub-aquatic mountain range far bigger than anything on the surface.

There is an awful lot we don’t know about what lies at the bottom of the ocean. That, in a sense, is the point of this latest mission to the Mid-Atlantic Ridge. Bram Murton, the chief scientist on what they are calling Project Ultra, casts it as a kind of high-tech mystery story.