One of the many pressing mysteries surrounding the disappearance of flight MH370 is the question of how a plane could have been diverted without contact from any of the passengers. Even 12 years ago, on 9/11, passengers on hijacked planes were able to make outgoing calls during their flights. But with MH370, there don’t appear to be any call records originating from the plane, nor are there any social media postings either.

There are a few possible reasons for this. It’s possible that the plane’s altitude—as high as 45,000 feet—made it difficult to get a signal from a cell tower.

Even at low altitude, tower signal density is low along MH370’s flight path. Tower signals spread and grow weaker as they travel further. Couple that with the high altitude and velocity of a passenger plane and that makes it very difficult to get a strong or consistent signal.

On social media, no mid-flight interactions have emerged. Some pre-flight posts have possible been unearthed on Weibo, a Chinese network. According to one site, “26-year-old passenger Yue Wen Chao posted a picture of his girlfriend and this note, ‘See you in Beijing.’”

Even if cell phone signals were stronger, that doesn’t do much for a plane full of unconscious passengers. If the cabin depressurized quickly, a lack of oxygen would have made it nearly impossible for passengers to remain awake.