He claimed to have invented a Death Ray that could destroy an entire squadron of airplanes with one shot, but he died before he could really explain how it worked or if he even really had invented such a ray in the first place. No prototypes or notes where found on the subject after his death. Some people think that if he had in fact invented a ray like this, it would have been seized by the military on his death. In fact the Office of Alien Property did seize his papers after his death, despite the fact he was a citzen of the US.
Tesla also said he could transfer energy anywhere in the world without the use of wires, but never said how that would be achieved. Although some people point that the Tunguska Blast of 1909 may have been the results of one of his failed experiments with that technology.