Understanding how magnetic reconnection can be fast even when the Ohmic diffusion time is many orders of magnitude longer than the dynamical time is a critical issue in reconnection research. Although much work on fast reconnection has focused on collisionless or kinetic effects, a number of recent theoretical and computational studies have shown that even MHD, or fluid theory, admits fast reconnection. We identified a secondary tearing instability that causes an initially uniform and thin current sheet to break up and reconnect more rapidly, at a rate that appears insensitive to the Ohmic diffusion rate. The time progression of the instability is shown below in the figure below.