Worry's about somebody covering the air intake and...
didn't the Waco Kid bury a school bus and they filled
it with fire?
They sell an aliens like auto sentry that will pop up out of the ground and get the guy trying to mess with your air. They also sell an rc heli too. if you look at the site, the bunkers they build are obviously thicker , they foam crete them in, and they have the required bracing for burial. 400k is still a lot for such a thing..you could have it built by a steel guy onsite fir less.