One of the many conspiracy theories about the 9/11 attacks is that the jets that flew into the World Trade Center towers, and the Pentagon, were not manned at all, but were actually un-manned drones.
One of the theories is that the planes had all been shot down somewhere in the Atlantic ocean and quickly replaced with the un-manned drones, or that they had been secretly landed somewhere, and that the crews and passengers had simply been made to "disappear".
The biggest problem with both of these theories are that not only would they require thousands of people who work for the NTSB and various airport air traffic control unions to be in on the conspiracy, it would also have to get perhaps thousands of other people within the military and the government to never talk about it to anybody. This is so improbable and illogical, that many people considered both theories to be theoretical impossible.
Also, in regards to the theory that the planes had been shot down and replaced with drones, even if the government had managed to get thousands of people to basically keep silent on this, the odds of no body actually witnessing any of the planes being shot down, even over the ocean, is really very low, and eventually debris from the planes would have washed up on shore.
Of course, debris washing up on the shore would have been very difficult to happen in the first place, since none of the four planes that were hijacked ever flew over the ocean. In other words, it would have been impossible for no one to have witnessed any of the four planes being shot down, crashing to the ground, and quickly being replaced with drones.
Another problem with this theory is United Airlines flight 93, which crashed into the middle of a field in south-western Pennsylvania, and not into a major government building. If it was a drone, and un-manned, then why didn't it hit any important buildings, and just crashed into the ground? Also, why were bits and pieces of human remains, and the type of humans remains you would find at a violent, high speed plane crash site, found at the crash site?
Besides the fact that this theory ignores that there are no eye witnesses to any of the planes being allegedly shot down, it also ignores the fact that phone calls from people on board those planes were made to loved ones on the ground, telling their loved ones that the planes they were on had been hijacked, and that some of them had even called their loved ones very soon before impacting the World Trade Center towers, or the Pentagon, or that field in Pennsylvania.