Infrasound as a scientific explanation for ghosts

Most people are aware of infrasound as the ultra-low-frequency sound used by elephants and whales for communication. It is also reportedly capable of affecting humans, including by producing visual phenomena:

Research by the late Vic Tandy, a lecturer at Coventry University, suggested that the frequency 19 Hz was responsible for many ghost sightings. He was working late one night alone in a supposedly haunted laboratory at Warwick, when he felt very anxious and could detect a grey blob out of the corner of his eye. When he turned to face it, there was nothing.

The following day, he was working on his fencing foil, with the handle held in a vice. Although there was nothing touching it, the blade started to vibrate wildly. Further investigation led him to discover that the extraction fan was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye... This was why he saw a ghostly figure — it was an optical illusion caused by his eyeballs resonating. The room was exactly half a wavelength in length, and the desk was in the centre, thus causing a standing wave which was detected by the foil.

Tandy investigated this phenomenon further and wrote a paper entitled The Ghost in the Machine. He carried out a number of investigations at various sites believed to be haunted, including the basement of the Tourist Information Bureau next to Coventry Cathedral and Edinburgh Castle.

More information and references at the Wiki link. I believe I remember hearing reports of governments or military forces studying infrasound as a tool for urban warfare and crowd control. Via.