Ask someone from Monroe, Michigan, south of Detroit, about the big convent in town and they'll tell you about IHM. Officially it's Sisters, Servants of the Immaculate Heart of Mary, an institution that has a worldwide reach with roots dating to Monroe in 1845.

The Time Capsules of Monroe County Michigan Facebook group brought to light recently a video tour of tunnels abandoned under the convent complex filmed in 2013.

The IHM campus is located across from the River Raisin and includes a Motherhouse and vacant school, St. Mary Academy.

Comments on Facebook share when the tunnels were in active use.

Of course, like many tunnels, the use was utilitarian and supported the infrastructure of the complex:

These tunnels were made to deliver steam heat from power house to Academy, Mother House and Hall of Divine Child buildings. Steam Lines, Water lines and pumps were located in them. Went through most of them in 60s.

This fascinating story says there was a chapel as part of the tunnels:

St Mary's Academy had a chapel in the tunnels, we would go there and also there were connecting ones that would go under Elm street to the river. A lot of tunnels in all directions. I was late for mass when I got lost. I got in trouble by the nuns.

More stories indicate the tunnels run to neighboring buildings and perhaps to downtown Monroe as well.

Take a look under IHM in Monroe:

Tunnels under American cities is a persistent and particularly fascinating urban legend. Here are more unsolved mysteries we can't stop thinking about:

Leading Theories About D.B. Cooper and 30 other unsolved mysteries

Thanks to the American fascination with confounding unsolved cases, mystery is among the most popular genres of books, movies, and television. From heists and capers to murders and robberies, the world’s greatest unsolved mysteries spark media frenzies that grab headlines around the globe. Some cases compel so much public intrigue that the facts and theories surrounding them become the basis of books, movies, plays, and documentaries decades or even centuries after the cases go cold.

25 True Crime Locations: What Do They Look Like Today?

Below, find out where 25 of the most infamous crimes in history took place — and what the locations are used for today. (If they've been left standing.)

More From Y105