For the third day, search and rescue operations from 10 different countries failed to find any trace of the missing Malaysian plane that disappeared about an hour into a flight to Beijing. Here’s a map of the plane’s last known location: 

There are numerous theories as to why the plane disappeared, but nothing concrete. Here’s Reuters with one scenario:

A senior source involved in preliminary investigations in Malaysia said the failure to find any debris indicated the plane may have broken up mid-flight, which could disperse wreckage over a very wide area.

“The fact that we are unable to find any debris so far appears to indicate that the aircraft is likely to have disintegrated at around 35,000 feet,” said the source.

Asked about the possibility of an explosion, the source said there was no evidence of foul play and that the aircraft could have broken up due to mechanical causes.

Reuters also reports that two passengers on the flight used stolen passports to board, but that may not have any connection to the disappearance. From Reuters:

A European diplomat in Kuala Lumpur cautioned that the Malaysian capital was an Asian hub for illegal migrants, many of whom used false documents and complex routes including via Beijing or West Africa to reach a final destination in Europe.

“You shouldn’t automatically think that the fact there were two people on the plane with false passports had anything to do with the disappearance of the plane,” the diplomat said.

“The more you know about the role of Kuala Lumpur in this chain, the more doubtful you are of the chances of a linkage.”

While mid-air crashes are less common than problems during takeoff and landing, they do happen.

And this disappearance notwithstanding, airline technology has gotten safer over time. Here’s a chart showing the number of crashes, and resulting casualties, since 1970: