MEQUON, Wis. (AP) — While many Midwestern college students pile on layers to brave the frigid walk to classes in subzero weather, those attending one Wisconsin university on the windy shores of Lake Michigan can leave their coats in their dorms and take a much cozier trek.

Concordia University Wisconsin has nearly 4 miles of connecting tunnels and hallways that keep students out of the harsh winter elements. The elaborate underground system connects residence halls and academic buildings.

"Actually, it's pretty funny to see students walking around campus in flip flops and shorts," during the winter, said university spokesman Craig McCarthy.

Some of the tunnels date back to when the School Sisters of Notre Dame owned the 200-acre lakefront property before Concordia bought it in 1982, according to McCarthy. The Lutheran university, north of Milwaukee in the suburb of Mequon, has added to the maze of tunnels when constructing new buildings, including the School of Pharmacy, which opened in 2011.

"Honestly, I haven't been outside in two days," said junior Lindsey Weber, a resident assistant at Augsburg Hall. "It's been nice with the semester starting back up. Everybody has been able to stay warm unless they're a commuter or had to go out for something."

The tunnels can get a bit crowded at times, said Weber, 21, of Colgate, Wis. About 7,900 graduate and undergraduate students from 46 states and 28 foreign countries are enrolled at the university, which is a member of Concordia University System, a nationwide network of colleges and universities.

"It turns into like rush hour on the freeway," Weber said. "Everybody just lines up and goes."

The private, liberal arts school ranks 13 in a list of the 21 coldest colleges in the country, according to the website College Prowler, a resource for student reviews, majors and scholarships. The ranking is based on student reviews of weather, average high and low temperatures and average precipitation in summer and winter, the website says.

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Built 20 years before Concordia bought the property, the cement floor of the underground maze was conducive to bikes and roller skates the nuns used to navigate the long tunnels, said Lawrence Sohn, 76, who maintains the university's archives.

"This was all the sisters' idea in the first place," Sohn said.

Twenty-two of the 23 buildings on campus are connected and clustered in the middle of the sprawling property with parking lots and roads encircling the buildings.

The university used the tunnels in a promotional campaign several years ago, telling prospective students they could "come to class in shorts in January," Sohn said.

"The nice thing is," Weber said, "we never have to put away our summer clothes."