The ominously swelling torrent of grey water churns down the gully as onlookers wait to see how bad it will get.

First large rocks and then boulders the size of cars can be seen tumbling amid the growing cascade.

Suddenly an unstoppable wall of water and rock surges round the valley bend, obliterating a concrete bridge and roaring on down the valley.

The video captured recently in northern Pakistan shows the terrifying destructive force of a growing natural threat to millions of people.

Glaciers found in the country's awe-inspiring mountain ranges are melting as global temperatures rise. The meltwater collects as glacial lakes that are often only held back by thinning ice walls, or fragile earth banks.

When these barriers break, perhaps when pressure gets too much or an earthquake strikes, in an instant millions of tons of water can be unleashed into the populated valleys below.