Far more troublesome and dangerous are the "leads" (the whalers' term for lanes of open water), which are caused by the movement of the ice under the pressure of the wind and tides.

These are the ever-present nightmare of the traveler over the frozen surface of the polar ocean--on the upward journey for fear that they may prevent further advance; on the return journey for fear they may cut him off from the land and life, leaving him to wander about and starve to death on the northern side.

Their occurrence or non-occurrence is a thing impossible to prophesy or calculate.

They open without warning immediately ahead of the traveler, following no apparent rule or law of action.

They are the unknown quantity of the polar equation.
Sometimes these leads are mere cracks running through old floes in nearly a straight line.