New research provides aclue to unlocking an astronomical mystery that dates back to the 1960s.High-velocity clouds (HVCs) are concentrations of hydrogen gas found all overthe sky, sometimes in large complexes. They are defined by their anomalousvelocities, which are inconsistent with the regular rotation of the Milky Way. Theorigins of HVCs are a mystery, mainly because their distances are unknown. Mostmodels place them in the Galactic Halo, kiloparsecs from the Sun. But what ifHVCs, or at least some of them, were much closer? The HVC known as MI may bethe result of a supernova that took place about 100,000 years ago at a distanceof 163 pc. Low-velocity HI data show a clear cavity, a structure indicative ofregions evacuated by old exploding stars, centered on the spatial coordinatesof MI, (l,b) = (165o, 65.o5). The cavity is also visiblein 100 micron dust data from IRAS. The invisible companion of the yellow giantstar, 56 Ursae Majoris, may be the remains of the supernova that evacuated thecavity and blasted MI itself outward at 120 km/s. The mass and energy of MI are easily in line with what isexpected from a supernova. The diffuse X-rays seen by ROSAT provide evidence of ahot cavity, and the enhanced X-rays may result from the subsequent bow shock. Ironically,this scenario for MI only came together because we were exploring low-velocitygas in the direction of high-velocity clouds.