I know you've got atmosphere density modeled, with regards to speed, ballistics, etc. Do you have this "calculated" at below sea level alts? In the AH terrain editor you can make canyons and such, and can even make gorges that drop thousands and thousands of feet below sea level. Heck, you can even move the water level down (as far as I recall).
So, how does this affect flight? Do things slow down a lot? Air is much much denser, do engines get more boost? Bullets travel less before slowing?
Just curious.