Seventy-five miles long and covering over 1,350 square miles in area, the Hubbard Glacier is the largest tidewater glacier in North America. It is also one of the most impressive, a 300-foot wall of ice rising sheer and jagged from the ocean. It has been thickening and advancing toward the Gulf of Alaska since it was first mapped by the International Boundary Commission in 1895. This is in stark contrast with most glaciers, which have thinned and retreated during the last century. This atypical behavior is an important example of the calving glacier cycle in which glacier advance and retreat is controlled more by the mechanics of terminus calving than by climate fluctuations. If Hubbard Glacier continues to advance, it will close the seaward entrance of Russell Fiord and create the largest glacier-dammed lake on the North American continent in historic times.