For decades, winter visitors to Apostle Islands National Lakeshore on Wisconsin's rugged Lake Superior coast have marveled at the artistry that happens when water, waves and subfreezing temperatures converge, creating natural ice sculptures as artful as glassworks. The ephemeral event, when upstream rivulets flow into caves at the lake's edge and harden into blue-green stalactites anchored in a bed of clear Lake Superior ice, is so popular with tourists that the National Park Service maintains a telephone hotline to let people know when it's safe enough to make the 2-mile hike across the hardened lake to view what are called the sea caves. The event is also becoming increasingly rare. The ice hotline hasn't given an "all clear" for ice caving since 2009. That's because Lake Superior's winter isn't what it used to be.