The longest mountain range is under the sea

If I ask you for the longest mountain range on Earth, you'll probably think of the Andes or the Himalayas. Wrong: the biggest by far is the mid-ocean ridge, a chain of mountains some 65,000 kilometres long that snakes across the floor of every ocean.

Hardly anyone has seen it, because it's submerged, often kilometres deep. It formed, and is still forming, where the plates of the crust pull apart and magma rises to create brand-new seabed.

It's like a giant seam running around the entire planet, longer than the Andes, the Rockies and the Himalayas put together and then some.

The most impressive mountain range on Earth is on no hiking map: it lies under thousands of metres of water, growing a little every year with nobody watching.