People generally hear sounds between 20 Hz and 20,000 Hz. Infrasound is below that range. It is sound with a pitch below 20 hertz, lower than the sounds people usually hear. It is wrongly thought of as completely inaudible, because people can perceive it when it is strong enough. Infrasound can be sensed if it is strong enough, mostly by the ear. A sound at the edge of hearing is easy to overlook, so it makes a fair question for anyone who wonders why a room feels strange.

Infrasound comes from nature too. Volcanic eruptions, earthquakes, falling meteorites, tornadoes and lightning make it. A scientist named Vic Tandy traced eerie feelings in a lab to a faulty extractor fan that made very low sound. Tandy was one of several scientists working in a medical equipment laboratory when the strange feelings began. The cause he found was an ordinary machine.

In 1998 Tandy and Tony Lawrence proposed that a standing air wave of 19 Hz may create the sensations people take for a ghost. Tandy later reported that infrasound of about 19 Hz had been found in a Coventry cellar where people had reported odd feelings. He also noted that only a minority of people are sensitive to the effects of infrasound. That point matters, because a room with infrasound in it would not be expected to affect everyone who walked through it in the same way.

In 2003 a London concert played infrasound under some pieces, and neither the performers nor the audience were told which. That design lets a listener's own report be compared with what was really playing. Nobody in the hall could know in advance which pieces carried the low sound. An experiment like this tries to separate what a sound does to people from what people expect it to do, and that difference is the heart of a good investigation.

At Hampton Court Palace, people who already believed in ghosts reported significantly more unusual experiences than people who did not. About two thirds of the unusual experiences people reported at Hampton Court were a change in temperature. The visitors who took part were members of the public who chose to join, and they were tested in groups walking the rooms. Scientists in England and Scotland asked visitors to walk around places with a haunted reputation and mark where they felt anything unusual.

The Society for Psychical Research was founded in London in 1882, to study such reports with care. It grew out of a conference held in London in January 1882. Eleanor Sidgwick, a mathematician and later principal of Newnham College, Cambridge, was one of the Society's leading investigators. The Society's Census of Hallucinations asked thousands of people about unexplained experiences, and the SPR says it is still the largest survey of its kind.

Researchers note that surprisingly realistic imagery can be seen on the threshold of waking. Sleep paralysis is when you cannot move or speak as you are waking up or falling asleep. It can be scary, but it is harmless. During sleep paralysis some people feel as if someone is in the room, and the feelings can last up to several minutes. The brain is partway between sleep and waking at such a moment, and a feeling of company is something a brain can produce.

A hill on Mars looked like a face in one old photo because of shadows. The hill is 1.5 kilometers (one mile) across, and the Viking photo was taken with the sun about 20 degrees above the horizon. The speckled look of the first Mars picture came from bit errors in the data, made more obvious when the photo was enlarged. NASA says the hill looks like a face only in low-resolution pictures with very specific lighting. The hill has dozens of hills like it nearby, and sharper photos showed it is just a hill. Pictures can mislead, and better pictures can correct them.

Seeing faces in things that have no faces has a name: pareidolia. The brain area that notices faces gets busy when we only think we see a face, like one in a cloud. The visual system groups small parts, like a mouth and eyes, into a bigger picture. Babies are attracted to faces from birth, even though their eyesight is poor. One research team says the brain evolved to help us deal with other people, and that shapes how we see the world, so we see faces in objects. Carl Sagan and others suggested that spotting faces fast helped early humans stay alert to others nearby. This is an idea, not a settled finding. Reports of hauntings include seeing a figure, odd smells, sudden changes in temperature and a strong sense of presence, and each can be examined with the same patient questions.

Words to know

• infrasound: very low sound
• inaudible: cannot be heard
• census: big count
• pareidolia: seeing faces
• paralysis: cannot move