You are aware that large parts of the land masses of Greenland and Antarctica are well below sea level?

The ice is so heavy the ground has been depressed by it.

The highest Point of the Greenland Ice cap is just over 3200m (10,500ft) above sea level.

The Humboldt glacier. You probably can't find a length for it, as nobody is quite sure how long it it. It's an an area where the Ice cap descends to the sea, and thins as it does so. How long it is depends on where you draw the line of where the ice cap ends and the glacier begins. 500m thick, 1000m, 1500m? They'll all give