Stewart Tunnel, a former railroad tunnel that’s now part of a bike trail, has great ice formations that form from water dripping from its roof. The wintery works of art look as though they could blend in among the sculptures in a modern museum.

These formations are mostly stalagmites. The constant, consistent winds that whip through the tunnel carve them into fascinating, alien-looking shapes. The best time to view them is during January and February, when there has been a solid streak of subzero temperatures.

The tunnel’s worth exploring even when the frigid air isn’t carving obscure shapes into the chunks of ice. The 1,200-foot-long passageway’s shape is an engineering anomaly. Unlike most other rail tunnels, when you enter this one, you can’t see the light at the end because of the way it curves.

It was completed in 1887 to serve a midwestern train line. At the time, the tunnel’s unusual shape was an impressive feat. It was named after James Stewart, the contractor who oversaw the project, who sadly died when he was thrown from a buggy while following the railroad’s planned route.

After the train service was disbanded in the late 20th century, the Department of Natural Resources hatched a plan to convert the unused track into a hiking and biking path called the Badger State Trail. After it opened, Stewart Tunnel quickly became one of its keystone attractions.

Update as of February 2021: The tunnel entrance is currently closed.