One place I find mysterious is the Old Minnehaha Courthouse museum. Some people say it's a haunted museum. And it is one of the most mysterious and creepy places in Sioux Falls. The museum is said to be haunted by various different spirits making it one of the most haunted places in South Dakota. Some people have claimed to hear someone fall down the stairs but once they checked, nobody was there. There are also disembodied voices and spirits. Staffs and workers at that museum have went through this mysterious and creepy experience. Some visitors have also claimed seeing a man sweeping the museum and once they approached him he just disappeared. Going to this museum would be terrifying but it would be fun going there if you like the adventure.