The Universe could be spatially flat, positively curved or negatively curved. Each option has been popular at various times, partly affected by an understanding that models tend to evolve away from flatness. The curvature of the Universe is amenable to measurement, through tests such as the determination of the angles of sufficiently large triangles. The angle subtended by the characteristic scale on the cosmic microwave sky provides a direct test, which has now been realized through a combination of exquisite results from a number of CMB experiments. After a long and detailed investigation, with many false clues, it now seems that the mystery of the curvature of the Universe is now solved. It is an open and shut case: the Universe is flat!