If you’ve ever wondered how Gymkana’s gravity- and fire-defying acrobatic stunts and tricks came to be, the answer is practice: 70 years of it. The troupe, which thrills crowds at Maryland Madness, local schools, military bases and even “America’s Got Talent,” is celebrating its anniversary with a new exhibit at McKeldin Library, which runs until August. Here’s a peek at some of the archival images: