What Causes a Flight to Vanish From Air Surveillance Radars?

Every commercial plane has something called a transponder, which is a transmitter and responder. So that's how the radar in these air traffic control towers knows where each plane is. In a military radar, you don't look at the transponder response or any of those things. You simply send a radio transmission, it hits the plane and the waves reflect back of the skin of the plane. And then it goes back to the radar and you can see where the aircraft is. It looks like the plane would have gone southwards and made its way into the heart of the southern Indian Ocean. There is no place to land over there, which means it would have most likely crashed.