In the White Mountains of northern New Hampshire, there are caves, tunnels, fountains, slides, and sculptures made of icicles. The dripping, organic shape makes them look like those dribble castles you make from wet sand at the beach, and at night they are lit up with an otherworldly glow. These towering human-made stalactites are known as the Ice Castles.

The Ice Castles project originated in Utah. It was started by Brent Christensen, who built an ice cave in his front yard to entertain his six kids. The structure was a hit, and has grown from a simple structure into a large-scale operation.

Each year, the creators go to wintry locations including New Hampshire, Colorado, and Wisconsin to start growing the castles. A series of drip pipes start the process of trickling out their icicles—the “seeds” of the castles. These icicles then form the basis of the structures, which are made entirely out of ice and snow.

Around 12,000 new icicles are formed and added every day. Team of artisans sculpt and coax the ice and snow into a system of tunnels and caves. By the time it’s done, a single ice castle weighs more than 20 million pounds. Inside the castles, visitors will find ice-carved tunnels, fountains, and slides. At night, the castles glow in pastel hues thanks to LED lights inside the frozen walls.

Depending on the weather, the castle typically opens in January and stays up through March, when the icicles start to melt and all the pipes are packed up.