One might think that with the extremely low temperatures near the North Pole, the ice must be hard, thick and smooth - making travel over the Arctic Ocean quite easy. Not so!

The ocean is up to 3 miles / 4.8 km deep in some places, and currents cause constant movement and changes on the surface ice. This movement pulls sections of the ice cap apart, creating open lanes of water called "leads".

For anyone travelling across the ice, a stretch of deep open water in -40° temperatures (C or F - take your pick) is a formidable obstacle. Anyone slipping into a lead could drown, or quickly freeze to death.

These channels can open suddenly and without warning, so much so that some early Arctic explorers would not sleep in sleeping bags for fear of drowning if a lead opened up while they slept.

And there are more obstacles - closer to land, where the land or land-locked ice interferes with the movement of the ice cap, steep mountains of ice called "pressure ridges" rise up to block the way. It's tough going out there!