The world’s biggest (not deepest) canyon has been discovered. Now, how did we miss that?

It was covered in ice…

Huge canyon discovered under Greenland ice

Scientists are surprised the feature has not been worn away by successive glaciations

One of the biggest canyons in the world has been found beneath the ice sheet that smothers most of Greenland.

The canyon – which is 800km long and up to 800m deep – was carved out by a great river more than four million years ago, before the ice arrived.

It was discovered by accident as scientists researching climate change mapped Greenland’s bedrock by radar.

The British Antarctic Survey said it was remarkable to find so huge a geographical feature previously unseen.

The hidden valley is longer than the Grand Canyon in Arizona. It snakes its way from the centre of Greenland up to the northern coastline and before the ice sheet was formed it would have contained a river gushing into the Arctic Ocean. Now it is packed with ice.

The ice sheet, up to 3km (2 miles) thick, is now so heavy that it makes the island sag in the middle (central Greenland was previously about 500m above sea level, now it is 200m below sea level).

The canyon still runs “downhill”, though, and meltwater from the ice sheet seeps out below sea level at the northern end – at a relative trickle, rather than a torrent. Glaciologists think the canyon plays an important role in transporting sub-glacial meltwater produced at the bed towards the ocean.

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