According to AccuWeather, mammatus clouds form on the underside of a cumulonimbus, they can develop underneath cirrocumulus, altostratus, altocumulus and stratocumulus. Basically they are the result from the sinking of moist air into dry air, so they are upside-down clouds.
After the storm the sun came out, the mammatus clouds formed, and people went outside to take photos of them. Take a look at some of the sights around Minnesota last night.
Reddit user dadykoff posted this photo.
Reddit user Scandanasian18 posted this photo from St. Paul