Center with u.s. and canadian operators standing watch. >> it's daunting. it's a little bit high stress. it's a very busy environment. but we train to that. >> this mountain complex was engineered to withstand a direct nuclear attack. and integral to that are a series of blast doors three feet thick. the last time they were closed operationally was on 9/11. >> it's all self-sustained with internal water, electricity, and food. >> the chinese balloon was determined not to be hostile, but it joins a list of growing threats from hypersonic missiles to cyber that norad is racing to stay ahead of. ...