Gravitational Storms

The aliens used gravity manipulation to carry their ships through our skies. That manipulation left lasting scars on the stability of our world. Now, gravitational anomalies can be found all over the world. Sometimes, the gravity in a certain area will completely invert itself, and pull everything off the ground, and into the sky, only to come back down with great destructive force.   While very few have ever touched a safe zone, there have been occasions, mostly following an airborne mechanon attack. After the first one nearly wiped out a safe zone, we have developed ways to survive, and minimize damages. Such methods include the invasion bunkers under every city block. The buildings within the safe zones are bolted to the ground along with most of their contents. The roads are magnetic to the point of pulling all vehicles firmly to the ground. Still, these storms remain a threat simply because they are unpredictable.