Microbursts are believed to have been responsible for several aircraft accidents in recent years. We report here laboratory experiments which show that when a descending column of dense fluid reaches the ground and begins to spread out horizontally, an intense vortex with a horizontal axis forms at the leading edge of the outflowing air. The intensity of this vortex results from the increase in vorticity due to the rapid stretching of the length of the leading edge. The properties of such a horizontal vortex with its associated updrafts and downdrafts are consistent with those found in microbursts which are a severe hazard to aircraft when taking-off or landing.