It has been so cold for so long in the Upper Midwest of the US that the Great Lakes are almost completely covered with ice.

At Apostle Islands National Lakeshore, in Wisconsin, the rock-solid ice has allowed thousands of visitors to trek over Lake Superior to explore caves featuring dazzling ice formations. It's the first time in five years the lake surface has been firm enough to allow passage.

With no letup in the cold, the caves haven't experienced the usual thaw-and-freeze cycle, resulting in icy stalactites dangling from cave ceilings like giant chandeliers.

Sections of the lakes, which hold nearly one-fifth of the freshwater on the world's surface, freeze almost every winter. But over the past four decades, the average ice cover has progressively receded by 70%, scientists say, probably in part because of climate change. However, as this season shows, short-term weather patterns can trump multi-year trends. Winter arrived early and with a vengeance and refuses to loosen its grip.