The Apostle Islands National Lakeshore includes 21 islands and a 19 kilometer (12 mile)-stretch of coastline. The region is known for having scenic cliffs and sea caves carved by waves crashing into rocks over thousands of years. When temperatures drop below freezing, water dripping inside the caves solidifies. Before long, huge icicles and ice sheets adorn the caves’ ceilings, floors, and walls.

Most people visit caves located along the shoreline on the mainland. To reach this area, visitors must walk on the frozen lake for about a mile. Some hikers trek up to 24 kilometers (15 miles) across the lake to reach ice caves on the islands.

For visitors to walk on the lake, the ice has to be strong enough to support their weight, says Neil Howk, an educator at the Apostle Islands. Unless the ice is at least 20 centimeters (8 inches) thick, it isn’t safe for hundreds of people to cross. “We need some really cold nights to jump-start the freezing process,” Howk says. “If we get a week where we have lows below 0° Fahrenheit, that’ll make ice in a hurry.”

But it doesn’t always happen that way. The caves were closed every winter from 2010 to 2013 because the weather was too mild, and the ice on the lake was thin. Two winters ago, when the ice was once again thick enough for visitors, nearly 140,000 people turned out from January through March. It was the most visitors the national lakeshore had ever seen in the winter, Howk says. 

The caves’ geology is a big part of what makes them such a spectacular attraction. The islands are made out of sandstone, a rock composed of layers of sand-sized minerals. 

Sandstone is porous, meaning it contains many tiny holes. The holes, combined with unusually thin layers of minerals that form the sandstone, make it easy for water to seep through and break down the rock. That’s how the caves formed, says Ed Nuhfer, a geologist and author of a guidebook about the islands. 

The sandstone’s porosity also allows icicles to form (see How Ice Caves Form). Most of the water dripping inside the caves isn’t lake water. It’s actually groundwater. This freshwater started out as rain and seeped down through the caves’ porous sandstone into the caves. When temperatures drop below freezing, the dripping water freezes into ice formations.

The icicles start out small and grow longer as water trickles down them and freezes onto the ends. Ice can also form on the sides of icicles, adding to their thickness. They continue to grow until their water supply is cut off or they get too heavy and break, says Stephen Morris, a physics professor at the University of Toronto in Canada.