Mixing under low to moderate Reynolds numbers is investigated experimentally in a stirred tank. An acid-base neutralization reaction and a pH indicator are used to reveal the existence, location, and size of segregated regions in the Bow. Such regions are observed for both unbaffled and baffled vessels stirred by either radial or axial flow impellers. These segregated regions have a complex internal structure consistent of several families of helical filaments wrapped around a central torus. It is demonstrated that the segregated regions are readily destroyed through the use of time-dependent RPM.