Bomb site at the cellar would be good, because then you could play with effects, like "ceiling blows and cars drop from the upper level of the park to the bottom..." etc...
ive been showing my clan the map and they have come up with several ideas:
1. Having some offices behinf the underground carpark based on tax fraud etc.
2. Some Sewers which lead you thro pipes eventually into a bomb site and into the underground park etc.
3. Having another building, which we dont know for yet.